I'm sorry, I dont know which stat scrolls use if you have a multiclass character.
Well, for spells directly cast by the character, it obviously depends upon what class has prepared this spell.
And if you have a singleclass character, its the Spellcasting Ability of that class:
Int: Wizard, Fighter, Rogue
Wis: Cleric, Druid, Monk, Ranger
Cha: Bard, Paladin, Sorcerer, Warlock, Barbarian
Source:
https://bg3.wiki/wiki/Spells#Spellcasting_Ability_and_Proficiency(Yes I didnt knew that either, I really never use scrolls, except for learning spells on a Wizard or selling them to get money)
But multiclass characters, I dont know and I cant find any clarification about this anywhere.