To be fair, when introducing new players to 5e the magic system does cause some confusion. And I'd expect a lot of people who never played 5e will be playing BG3.
When you have a full caster things are straightforward, but then you have 1/2 and 1/3 casters. And then you have warlock spell casting.
Add multiclassing on top of that (they were planning on that, right?) and you have an even more confusing situation where you have X level spell slots while not being able to prepare spells of X level.
When you are playing 5e at the table the player/DM will explain it and answer questions.
When you are playing a video game you expect the game to explain this to you. And currently BG3 is not doing a great job at it, I think.
