Detect Magic would cause an aura to appear around a creature/humanoid in polymorph form, yes?

I believe that's the case both RAI & RAW.

What about shapechangers? I watched Jeremy Crawford’s sage advice about it, but am still unsure. 

The spell's description states, "The spell has no effect on a shapechanger or a creature with 0 hit points." Please clarify if you meant something else.

Thanks for the reply!

My intent was not clear, I meant if a shapechanger changes shape, is it detectable with Detect Magic?

In my game, I would rule that detection of magic on a shapeshifter would depend on whether the ability is natural or magical. If the shapeshifting is an intrinsic aspect of the shapeshifter, then no. If the shapeshifting based in a magical ability or power, Detect Magic would detect the shapeshifter, but only while the shape shifting was occurring, not before or after. For example, though a wizard has magical abilities, Detect Magic wouldn't detect the wizard, but would detect the wizard's magical items or ongoing magical effects. At least, that's my take. I don't play in Adventurer's League (AL), so I don't worry too much about rules as written (RAW).

On the other hand, I can also see how using Detect Magic to detect magical beings could make for a fun in-game element. In the end, if you're not playing in AL, what would you and your characters enjoy the most?

I would expand this a little. If the spell is a concentration one, like Polymorph and True Polymorph are, then I’d say Detect Magic should give away that there is an active magical effect. On the other hand, Disguise Self is not a concentration spell, so I’d say you can detect it only when cast, not when active.

That would be a horrifying yes, "creature.. into creature"

so hear me out if you use polymorph on any creature turn it into a caterpillar and make a super tiny wall of force bubble around it could you instakill it?

edit: aside from really small creatures which, let's be honest, you probably won't be fighting by the time you unlock both of the spells

if I cast this on myself ... is there any issue with concentration given that I suddenly have a very low intelligence of (maybe as low as 1 or 2)?

Quote from Schmicol >>

if I cast this on myself ... is there any issue with concentration given that I suddenly have a very low intelligence of (maybe as low as 1 or 2)?

 
Nope, but a DM may prompt you anytime anything vaguely distracting happens. Prooobably more of a Wisdom issue; low-int just means that you don't have the ability to learn, recall or apply knowledge, not that you can't maintain focus on a task (see: apex predators).

To clarify, there's no as-written restriction, but you'd need to act in-character accounting for your new form's stats. So if you are a Wizard, I'd argue that you're used to relying entirely on your intelligence and wouldn't have much of your old self, while a Bard probably would rely more on their force of personality and spontaneous whim to determine who they are and thus maintain that connection. Ultimately, it's up to the established character, I suppose, and bear in mind that /as the caster of the spell/, any level of mental acuity will be entirely aware that you /want/ to be this shape... until of course, you don't.

Yes

When it says equal to or less than the challenge rating does it mean the easy, average, or hard challenge rating? 

Hmmm... what do you guis think happens to a big creature that has swallowed you if you polyporphet it (from the inside^^) into a ....bunny ??

There are so few beasts after rating 6

Most games I have seen and played would just rule that you are puked out as a matter of course. Maybe some damage on the way out, but it depends on the DM

The question is, if you are inside of it, who is casting the polymorph? If it is you (inside) I would argue having to roll a con save the second you are ejected (assuming the polymorph worked on the creature you arw inside).

Remember, con saves for concentration are not exclusively for taking damage. Any circumstance the DM thinks would be appropriate could cause one.

When im polymorph...can i still use my feat to have advantage on cons save to keep the form?...or i have no feat at all?

Quote from Zenith360 >>

So if your level 18 can you polymorph into a Beholder??

 If you use true polymorph i geuss you could....then have the person look into a mirror they see they are a beholder they now believe they are a beholder due to the way they are described in the MM (their mind changes reality) you can safely end the spell and they will still be a beholder

Some beasts have hands. Apes, for example. Parrots and Ravens can speak. According to this writeup, a person polymorphed into an ape cannot climb. I'd say this needs to be revised by WoTC to accommodate for sanity. (a losing proposition, they don't seem to care much about that)