When PC chooses Mark as their Action

Is this Action automatically combined w/ a Melee Attack?

Where are you getting the Mark action from?

DMG

The Mark rule is an optional rule described in the DMG in the Dungeon Master's Workshop here: https://www.dndbeyond.com/sources/dmg/dungeon-masters-workshop#Mark

Using the Mark rule to mark a target has no action cost at all, it is just something a player can choose to do when they make a Melee Attack. This can be from taking the Attack Action or from an Opportunity Attack, how you make the Melee Attack doesn't matter. The Mark rule also doesn't require that the Melee Attack hits or succeeds, just that it is made. Also note this includes Melee Spell Attacks like Shocking Grasp, or a Grapple or Shove attack even though these use an opposed skill check rather than an attack roll.

Yes, you would Mark automatically as part of your Attack action or whatever caused you to make a melee attack. However, as that is an optional rule, ask your DM if they are willing to use it. By default, most people do not use the Optional rules.

My confusion of this rule come here:

”The attacker is limited to one opportunity attack per turn.”

Who’s turn exactly because a few sentences prior to this quote is this:

”Until the end of the attacker’s next turn,…”

To me this implies one Round

If the attacker can Multiattack wouldn’t this option Mark rule mean they can then Mark as many creatures they can Attack?

Then be able to make their Opportunity Attacks on as many creatures as they’ve Marked before their next turn…

Is this how it works?

the terms "one Round" and "Until the end of your next turn" mean essentially the same thing and are sometimes used interchangeably. For example, if you look at the spell Absorb Elements the listed Duration is 1 Round but the spells description tells us that it grants resistance ". . . until the start of your next turn."

Technically a Round begins at the highest initiative count and ends at the lowest. This is slightly different from "Until the end of your next turn" as if your turn is in the middle of the initiative order then the effect would be active during the last half of the first Round and the first half of the second Round.

As for characters that can make multiple attacks on their turn, yes they can mark multiple creatures. Also yes, if the character marked multiple creatures and the creatures each provoked an Opportunity Attack on their turn the character could make an Opportunity Attack against each of them with Advantage without using their Reaction.

Quote from eightyocho >>

My confusion of this rule come here:

”The attacker is limited to one opportunity attack per turn.”

Who’s turn exactly because a few sentences prior to this quote is this:

”Until the end of the attacker’s next turn,…”

To me this implies one Round

If the attacker can Multiattack wouldn’t this option Mark rule mean they can then Mark as many creatures they can Attack?

Then be able to make their Opportunity Attacks on as many creatures as they’ve Marked before their next turn…

Is this how it works?

Because the attack of opportunity granted by the mark doesn't expend a reaction, yes, that's basically how it works. But it's still important to note the limitation you're asking about: you only get one attack of opportunity per turn. Let's say you're a fighter with 3 attacks.

Your turn: You make three attacks against three enemies adjacent to you, Enemies A, B, and C and mark them each.

Enemy A's turn: Enemy A moves out of your reach, and you make an AoO against them.

Enemy B's turn: Enemy B move out of your reach, and you make an AoO against them. They then use some special ability that lets an ally use their reaction to move, and Enemy C also moves out of your reach. You can't make an AoO against Enemy C because it's still Enemy B's turn and you already made an AoO against Enemy B on this turn.

Enemy C's turn: It's a new turn again, so if Enemy C does something that provokes an AoO again (like move back into your reach and then leaving), you can NOW make an AoO against them.

Now I fully understand

Holy guacamole! 🥑 

You truly have to be a D&D attorney to fully understand these rules

What kept throwing me off was the aforementioned “…per turn” but I now see 

That’s not relegated to attacker’s turn… it’s any creatures’ turn before attacker’s next turn

This is nuts! 🥜 

Thanks all 🙏🏾 You’ve been very helpful bringing clarity to this rule

Wow, Mark could be devastating when used by a Rogue, since Sneak Attacks are similarly limited to once per turn - meaning potential Sneak Attack on each Marked creature drawing an OoA, as well as using the Rogue's reaction to OoA another creature. If Sneak Attack would apply in those cases.

Quote from naruhoodie >>

Wow, Mark could be devastating when used by a Rogue, since Sneak Attacks are similarly limited to once per turn - meaning potential Sneak Attack on each Marked creature drawing an OoA, as well as using the Rogue's reaction to OoA another creature. If Sneak Attack would apply in those cases.

Yeah I think some of these rules are optional for a reason (also, do you mean AoO rather than OoA?), this one practically gives characters legendary actions with no restriction on the type of attack they can make (legendary monsters usually only have one of their weaker attacks as a legendary action). It seems weird that it's not limited to one creature at a time, like every other "mark" ability is.

It feels like an ability you should bestow to a specific player as a reward, rather than something you should enable for the entire game. Similar to Disarm in the same section; there's little incentive to not try it on elite enemies with weapons if you have multiple attacks, and it's a nightmare for DM's who then have to work out what a creature's unarmed attacks should be like, it makes more sense situationally.