I have been reviewing True Polymorph recently and realized it leaves a lot of things unanswered.  Hoping to get some general consensus on these questions:

1. What happens when a True Polymorphed Creature goes to 0 and the species had on on-death ability. Specifically look at the Phoenix.  It explodes, leaves behind a cinder/egg which does more damage and turns into a new phoenix. Does that activate? Do you take the explosion damage? Does the cinder/egg appear? Do you become the egg? Does the egg damage you? Similar creatures include the arclight phoenix and the draconic shard.
2. What happens if you try to T.P. someone holding an artifact? Yes the spell says "The target's gear melds into the new form." but can that really remove the hand of Vecna or the Wand of Orcus?
3. Legendary Resistances.  The RAW says if you become a Legendary monster you do not get Legendary actions, Lair Effects or Regional effects.  But it makes no mention of Legendary Resistances.
4. While you cannot TP a shapechanger, the spell does not say anything about when you turn yourself INTO a creature that is a shape changer.  Can I turn myself into a Gold Dragon then use it's Change Shape ability to become a Red Dragon?
5. When I turn into a spellcaster that has spell slots, are they separate or do those count as mine?  If I have already cast my 8th level slot, can I  become an Arcanaloth (CR 16) and still cast it's Mind Blank spell? Or because I already cast it, is it used up.  Also vice versa - if I use it as an Arcanaloth is it gone when the TP ends?

Yes, I know "Ask your DM" but wondering how other people handle these things?

Let me say first that I have never had to deal with this in an actual game and this is just my off the cuff thoughts. That said, let's go through your questions:

1. For a Phoenix its Fiery Death and Rebirth triggers when it dies, not when it drops to 0 hp. So my answer depends on what exactly happened. If you were simply damaged then you would drop to 0 hp and True Polymorph would end. If you were killed outright by something like Power Word Kill then Fiery Death and Rebirth would trigger since you never dropped to 0 hp. If you were instead a Star Spawn Larva Mage with Return to Worms that also triggers on dropping to 0 hp then which triggers would be decided by the player controlling the creature with the current turn, as per XGtE.
2. I wouldn't handle a creature holding an artifact any differently. If the creature is intended to be a significant threat then it likely has a variety of defenses, like strong saving throws, legendary resistance, etc.
3. I feel like I am missing something because I don't see anything in the spell description of True Polymorph that references Legendary Actions or legendary creatures. So RAW I think you would keep both Legendary Actions and Resistances. Lair actions are not part of the creatures stat block so you definitely wouldn't keep those. I would probably rule you don't keep Legendary Actions or Resistances, but I don't think that is supported by the RAW.
4. An Adult Gold Dragon has a feature called Change Shape but it doesn't have the Shapechanger subtype like a Succubus. Also an Adult Gold Dragon's Change Shape feature us limited to Humanoid and Beast creature types and has other restrictions as specified in its description. I would rule that you can use True Polymorph to turn into a Shapechanger, you just can't cast True Polymorph on a shapechanger. Once in a form that has a feature that allows you to change your form, like an Adult Gold Dragon or a Succubus, you must follow the rules for that feature.
5. When you cast True Polymorph you effectively replace your character sheet with the creatures stat block. So you would have a fresh set of limited use features. When True Polymorph ends you return to your character sheet in whatever state it was in when you first cast True Polymorph. This framing is even used by WotC in the Sage Advice Compendium here: https://www.dndbeyond.com/sources/sac/sage-advice-compendium#SA276

Quote from Fangeye >>

Let me say first that I have never had to deal with this in an actual game and this is just my off the cuff thoughts. That said, let's go through your questions:

1. For a Phoenix its Fiery Death and Rebirth triggers when it dies, not when it drops to 0 hp. So my answer depends on what exactly happened. If you were simply damaged then you would drop to 0 hp and True Polymorph would end. If you were killed outright by something like Power Word Kill then Fiery Death and Rebirth would trigger since you never dropped to 0 hp. If you were instead a Star Spawn Larva Mage with Return to Worms that also triggers on dropping to 0 hp then which triggers would be decided by the player controlling the creature with the current turn, as per XGtE.
2. I wouldn't handle a creature holding an artifact any differently. If the creature is intended to be a significant threat then it likely has a variety of defenses, like strong saving throws, legendary resistance, etc.
3. I feel like I am missing something because I don't see anything in the spell description of True Polymorph that references Legendary Actions or legendary creatures. So RAW I think you would keep both Legendary Actions and Resistances. Lair actions are not part of the creatures stat block so you definitely wouldn't keep those. I would probably rule you don't keep Legendary Actions or Resistances, but I don't think that is supported by the RAW.
4. An Adult Gold Dragon has a feature called Change Shape but it doesn't have the Shapechanger subtype like a Succubus. Also an Adult Gold Dragon's Change Shape feature us limited to Humanoid and Beast creature types and has other restrictions as specified in its description. I would rule that you can use True Polymorph to turn into a Shapechanger, you just can't cast True Polymorph on a shapechanger. Once in a form that has a feature that allows you to change your form, like an Adult Gold Dragon or a Succubus, you must follow the rules for that feature.
5. When you cast True Polymorph to you effectively replace your character sheet with the creatures stat block. So you would have a fresh set of limited use features. When True Polymorph ends you return to your character sheet in whatever state it was in when you first cast True Polymorph. This framing is even used by WotC in the Sage Advice Compendium here: https://www.dndbeyond.com/sources/sac/sage-advice-compendium#SA276

 On point 3: The RAW on them losing Legendary Actions is in the Monster Manual, during the intro section talking about Legendary Actions, not the spell. But nothing about Legendary Resistances, so you do get those.

I believe an SA has confirmed you keep LR's as well. Which handily helps you keep up concentration on your polymorph.

Ah, thank you Athanar90! I did a little digging and the section in the Monster's Manual was added as an errata in 2018, I have a digital copy though so I just didn't think to check there XD.

As for the Sage Advice Compendium I did a quick search and didn't find any relevant entries on Legendary Resistances. There is this exchange saved on sageadvice.eu: https://www.sageadvice.eu/can-a-true-polymorph-use-legendary-resistances/

To summarize JC says that the errata rule added to the MM has no bearing on Legendary Resistances and then follows up saying that you don't get legendary resistances without providing any real explanation. So RAI at least is that you don't get Legendary Resistance, but I am not sure what rule JC was referring to when he says that you don't.

What happens when a True Polymorphed Creature goes to 0 and the species had on on-death ability. Specifically look at the Phoenix.  It explodes, leaves behind a cinder/egg which does more damage and turns into a new phoenix. Does that activate? Do you take the explosion damage? Does the cinder/egg appear? Do you become the egg? Does the egg damage you? Similar creatures include the arclight phoenix and the draconic shard.

What happens if you try to T.P. someone holding an artifact? Yes the spell says "The target's gear melds into the new form." but can that really remove the hand of Vecna or the Wand of Orcus?

Artifacts are not stated to be immune to magic or magical transformation, nor does the spell say it doesn't affect artifacts, so yes.

Legendary Resistances.  The RAW says if you become a Legendary monster you do not get Legendary actions, Lair Effects or Regional effects.  But it makes no mention of Legendary Resistances.

While you cannot TP a shapechanger, the spell does not say anything about when you turn yourself INTO a creature that is a shape changer.  Can I turn myself into a Gold Dragon then use it's Change Shape ability to become a Red Dragon?

Yes, for the reason you stated

When I turn into a spellcaster that has spell slots, are they separate or do those count as mine?  If I have already cast my 8th level slot, can I  become an Arcanaloth (CR 16) and still cast it's Mind Blank spell? Or because I already cast it, is it used up.  Also vice versa - if I use it as an Arcanaloth is it gone when the TP ends?

Wow, everyone basically agrees.  Thanks for the help