It ranges heavily. Unity used to have the majority of uses, but Godot has been slowly gaining more traction. There's no mandate for an engine, as I've worked with Unity and Godot programmers. Two of the projects I helped with were made in GameMaker. It comes down to what you're most familiar and speedy with during the typical 3-Day time period.
Huh, interesting. Most of the programmers I've worked with are in that same boat. Ideally, many prefer to work in pairs (unless their work ethic is absolutely godly), because splitting the workload eases the process, especially when there's not a lot of time to work with. Setting up a GitHub repository during the planning phase is often needed with that.
Designer is an important role that often gets absolutely shafted in my opinion, and even if you get to do Design, you are never credited for something so invisible to Players. But like, you aren't really allowed to be solely a Designer unless you have another craft from what I've experienced, same usually applies to the Writer role for some reason (the Artist usually takes the Writer's role, and results vary drastically). The Designer role is often forced onto the Programmer whether they want it or not, because many times the thought process from the rest of the team is typically: "Programmer = Designer".
There are some things that carry over, but I've worked with Programmers that don't want to do game design (it's extra work with time they don't have), and those who add everything in their head before asking themselves if it's even a good idea or not. It's the latter that eventually made me fed up with constantly finding a team to join, and why I prefer to organize and plan out things myself nowadays.
I'm mainly a Composer, but I have done Design for teams accepting of input, because I'm still attaching my name to something... and, you know, I want that something to be... good? Or, at the very least, be happy on how it turned out.
Thanks, I'm usually not very comfortable sharing my experiences in Jams, but I can tend to get over it if I'm feeling passionate enough about the topic. I know I'm probably not allowed to speak my mind where discourse about them are held, because I'd be browbeaten into accepting the status quo. Being critical often burns bridges existing and upcoming, and makes you the local outcast everyone ignores when all you want are for things to get better.
Honestly, I think it'd be neat to work with you someday. Also, the current ongoing Jam just ended, so I might end up seeing the rest of you someday for the next. Who knows?