The 5E rules are kinda confusing,
Eh, not really. "Never allow two (non-cantrip) spells in the same turn" is as plain and inequivacable as it gets.
It’s actually not that simple. 5E won’t let you cast Shillelagh (which is a BA cantrip) and a levelled spell in the same round.
Or players might quicken a cantrip to a BA thinking they’ll use their action for a spell, then wonder why they can now only cast a second cantrip with a casting time of one action? It is one of the most kind-bending rules in 5E for new players. The way you’ve boiled it down to one phrase how you did shows you don’t fully understand it either.
