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If a character or creature has an AC that with all bonuses requires a 20 to hit does that mean that every hit is a crit? Or alternately the creatures AC is so high that with all bonuses no hit would be possible except that a 20 always hits. So if a nat 20 is rolled you hit and crit and then the only way you can hit the impossible to hit creature also results in a crit.
I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit. What is correct?
You've written:
So if a nat 20 is rolled you hit and crit and then the only way you can hit the impossible to hit creature also results in a crit.
That's right -- you've understood the rules correctly.
You've also written:
I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit.
That might be a good house rule, but it's not a part of the official rules.
If you run a game, you could make that be a house rule. You probably shouldn't, though, because:
Very few monsters have an AC that is that high. The highest I could find was the Tarrasque (CR 30) with an AC of 25; the second highest was the Warforged Colossus (CR 25) with an AC of 23. Most fourth-level characters will have +6 to hit, so unless you're running a story where third-level characters fight the Tarrasque, this situation should never come up.
From the rules for attack rolls (emphasis mine):
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit, which is explained later in this section.
A critical hit totally disregards armor class. Since this hit is called a critical hit, we consult the rules for critical hits:
When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.
The intro the the Dungeon Master's Guide says:
The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.
If you are the DM and you don't like this particular rule, you have the liberty to change it. But as with all houserules, discuss it with your players first.
I was going to make this a comment, but it became more involved...
In the comments you gave an example:
If a creature's AC makes it impossible for you to hit it with all mods and bonuses but a natural twenty always hits I feel like the natural twenty in this case only grants you the hit not the crit since you are only able to hit it because a nat twenty always hits. Example: a first level fighter with a 16 strength using a long sword comes upon a creature with a 25 AC (not likely but bear with me) the only way for the fighter to score a hit is a twenty +3 for strength +2 for Proficiency = 25. so its a crit every hit seems odd
But that's not an "impossible" AC to hit. It is only very hard.
If that fighter also had a Bless spell on them, it would give an additional d4 meaning they could potentially hit on a 16! So long as they also rolled a 4 on the Bless die roll. Or someone gave them a enchanted (+1, +2, or +3) weapon. Or they were using a bow, had a 16 Dex, and had the "Archery" fighting style for a +2 bonus.
It just so happens that for a given scenario, to hit a certain AC, it requires a 20 plus all of a players' current bonuses/mods is just math. The crit is still a crit.
