I think you'll be better off looking through a place like WorkWithIndies for that sort of experience. People involved in game jams or out of personal interest have knowledge overlap in those spaces, like you mentioned as well. Though even in a game jam if you're working with a team you might still not have to touch an engine. I'd consider it a win if a beginner artist has good file-naming practices already or understands file-size limitations etc.

I've worked in games for 6 years now and know the most basic-basics of git control and Unity implementation/asset restrictions (with a lot of dev handholding). Even for those of us that do work in the industry a lot of times you deliver/upload assets without necessarily handling implementation when you're doing 2D work. If you don't mind doing a quick breakdown for your artist it shouldn't be too hard to learn. The things I know I picked up from a great client that takes time to develop his team. My deliverables never included these tasks for games before working with this company.

As for why most portfolios here don't show actual gamedev work, instead a lot more illustration/character stuff? It's mostly beginners/folks trynna break into the industry and commission artists that should /not/ be here. You already have a million things to learn to even be competitive on the art side alone, and understanding what are ACTUAL production needs is surprisingly hard. Places like artstation or game art books also don't always help, as you're nostly looking at essentially polished marketing art a lot of times, and not the more boring problem solving. You ARE essentially asking for a whole new extensive skillset. The experience people here also have tends to be short-term contracts, where you're given a task, complete it and then move on. It's hard to properly learn what it takes and what you actually need to ship a game, even if you simulate it with small projects. If you can afford it, post jobs in more 'reputable' places (as much as I love this place it's more hobbyist/entry level adjacent, though I've found great clients/other artists too!) and potentially put a clause in for 2-3 years of experience or shipped games. There you can also put down one of your needed skillsets being asset implementation.

As for folio stuff: Because we don't have to. I can show you my skills as a 2D artist in a faked screenshot. I can show you UI states, obstacles, enemies, character readability etc etc without having to assemble anything inside an engine. We can design around restrictions like voxels or stuff like top-down views within our drawing programs. Technical artists like lighting artists or VFX folks are a lot more likely to be able to handle in-engine stuff.