How many CR 11+ monsters have no resistances? I don't know the answer to that, but, I'm pretty willing to bet, "Not many" is pretty close. IOW, virtually everything you use in an encounter in double digit levels will have resistances. And typically multiple resistances.
Without looking it up, what resistances and vulnerabilities does a black Slaad have? Nalfeshnee? Gelugon? You know these off hand? Because as a player, if I'm not calling out my different damage types, I won't know which ones are relevant or not. So, they all need to be called out.
None of these questions are at all relevant, and the last line is simply untrue. Since you don't seem to be able to infer the way it works:
Player says "I'm doing X, Y, Z damage, do I need to separate it?"
DM looks at their reference (typically a printed stat block or saved copy in an online doc). Note that they need to have this at hand to know the monster HP, abilities, damage, and so on.
DM then says what damage needs to be tracked separately, if any. This is how the player knows which damage types are relevant.
Player then totals damage that doesn't need to be separated and each type that does, and says those. The player doesn't need to call out every damage type separately every time unless the creature has a really unusual mix of damage modifiers (something like vulnerable to fire, immune to thunder, normal from radiant, resistant to B/P/S).
For example, if fighting a devil with the damage types I listed before, the DM would say 'they're immune to fire, do others together'. If fighting a solar 'they're immune to the radiant, do others together'.
You're making all of this stuff out to be more complicated than it needs to be and...
You keep suggesting solutions to problems that I keep telling you I don't have.
complaining that the stuff you're choosing to ask questions about is irrelevant when someone answers you. I think it's absurd to say 'how can you possibly do this?' in one part of the post and 'oh the thing I was asking about really isn't a problem, why would you answer what I asked' in another. I answered the last one just in case anyone else is wondering how it works.