I don't know if I actually understand the question.
Are you asking, "Is it explicitly always in-story that every spell is an elective choice of the character?" Well...no, I wouldn't think so anyway. Or are you asking, "Is it even possible in-story that a spell could be an elective choice of the character?" Well...yes, I would consider that an absolute given, unless contextually forbidden. Yet those two answers seem trivially obvious to me, like asking "does EVERY soda have to be fruit-flavored?" or "can ANY soda be fruit-flavored?", so I feel like I have to be missing something really important that pushes this out of such obvious answers.
I'll build an example that might help get my point across, which I may not have explained sufficiently.
So imagine a character concept of an older "wizard" who is actually a scholar with not a whiff of magical talent, who "made a deal with the devil", gaining arcane power in the process. So tome fiend warlock.
The mechanical build is a blaster - EB+AB, fireball, that kind of stuff.
But here is where
who chose the spell "in game" (the warlock or the patron) becomes important. If the scholar is the one choosing their spells and they are taking blasting spells, it shows that this is what the scholar wants - to have the power to blow stuff up (vs more finess/utility magic).
However, if the patron is the one who chose the spells, then perhaps the relationship between the scholar and their magical powers becomes fraught. The scholar is dismayed, even afraid, of their own powers. Perhaps the scholar wanted tongues and hypnotic patterns, but instead got fireball and summon shadowspawn.
I mean you could say that the patron
and the scholar both want to blow stuff up, and in that case it doesn't matter who chose. But if the patron is the one making those choices, it opens up a different roleplaying direction.