When we create a char we start as lv1 char as one of the classes.
Does the game assume we had months or years of training before becoming a lv1 char?
A lv1 fighter is profient with martial weapons and heavy armor, a mage can read magic scrolls and so on. I guess it takes some time and effort to learn this and the average farmer or merchant will never learn this.
Maybe with the exception of sorcerers (who have literally magic in their blood) does the game assume you had a trainer, you were trained in a temple or something similar and does your relationship to that trainer (in a broader sense) play an important role in the game?
This question is more about DnD in general, not specific to this game.
It's usually assumed you've been trained, and some things (like a fighter reader scrolls) is partially rules and partially the dm (larian in this case) allowed leeway for fighters that might have a history with magic
The thing you 'need' to understand is the levels themselves (and by extension, increments) are abstractions. In Baldur's Gate III's case it's very deliberately vague because with post above with Mr. Mercer's video tries to explain, you can sit down and think of a backstory and then cross reference it with the DM, who can then go "No, X won't make sense in the campaign."
To return to the scroll, Tav could be a (body)guard in an enviroment with lots of magic, such as a mage school. They might not have any experience with scrolls at all. They might have been an adventurer for a while and learned-on-the-job. The game can't sit you down and go through every possible solution (and from my memories on Baldur's Gate I+II, it meant scrolls were basically 'free casts' for the mages only)
This abstraction also applies to levels: you don't suddenly multiclass from level 1 fighter to a level 1 fighter+level 1 thief and instantly get all the knowledge. Tav's been practicing in camp on lockpicking (perhaps with Asterion's help?). Shadowheart's been giving him tips on how to sneak around, ect. Going into that thief level is the culmination of all the practice, to the point Tav can reliably do those things without supervision.
I hope that explanation's made sense and helps
