Quick question that I am unclear on the answer. When you are using a class that has proficiency in medium or heavy armor, does that proficiency behave the same as the feat? I am trying to determine my next step in building a new character. Help is appreciated. Thank you in advance.

Can you clarify what you mean by "behave the same as the feat"? Both just grant proficiency in an armor type. The rules for that are described elsewhere, so I'm not sure what behavior you're talking about.

Proficiency is proficiency is proficiency.

For example, if you have a character that is proficient with light armor, and then takes the Moderately Armored feat, the proficiency granted by that feat is exactly the same as the proficiency with Medium Armor that would be granted by any class, subclass, or race. But not class, subclass, nor race would also grant the additional point to Str or Dex that the feat grants. And none of those would grant any of the bonuses included with the Medium Armor Master feat as that is its own completely separate set of bonuses.

That answers my question. Thank you.