Would things be that much different? Of course, if they used another framework than I would need to familiarize myself with it, but some don't use frameworks at all and build from the ground up (I'm assuming). Would people expect me to know how it works out the box?
I can't tell you exactly how different it would be, but even going from one game to another is very different and the logic sometimes flows differently, so I imagine yes, it will be a bit of a jump!
I'm an owner who hires out for all of my major programming, and my games are not built on a framework. If I were to hire you, I would absolutely expect you to know how it works out of the box. Every programmer does things differently, and even within the same languages they don't always follow the same syntax and logical organization, so taking an hour to two to familiarize yourself with the game's code is completely reasonable, but anything beyond that would make me question your ability to work with the code. Think of it like co-writing a book: you should be able to read what the other person wrote, and then jump in and continue the story, but if you have to look up every few words in the dictionary and pull out writing manuals and stylebooks to understand their story, then you're not ready to write in it.
Also, to add on to what
@Anoua said, you will largely be responsible for knowing where things are, because that's why you've been hired by the owners. Personally, I've been working with my games for a long time, and I have a basic~intermediate skill level in the languages my game is written in, but I don't do my own programming. So I can generally tell a programmer what script things are in/where they belong, and explain how things work, and usually also point out the spot thats causing issues in the case of a bug. However, that's not the case with every owner, and some of them will only be able to tell you what's wrong/what needs to be done from a frontend perspective, and you'll be expected to handle all of the backend components.
I hope that helps!