what size of startup should I target to learn how things work? I will not promote

Hey everyone, I'm 24 and just finished my master's in Computer Science. I've always been curious about starting my own company someday, and I feel like right now, being able to be financially supported for a while,might be the best window I'll get for taking that kind of risk. That said, I currently have zero real work experience, no co-founder, and no network in the space. So my plan is to spend this next year working at a startup first, to actually learn how the environment works from the inside and get some job experience. But I have a few doubts and would love some input. Given that "startup" can mean anything from a 2-person company to something the size of OpenAI, what size/stage would you recommend targeting so it's structured enough to actually learn how things work (mentorship, processes, real product-market fit), but not so early-stage that it's total chaos with no one to learn from? Also if during this year I worked on the side on a few of my own ideas (validating them, talking to potential users, etc.), do you think that's realistic to pull off alongside a full-time job? P.S. I posted about startups a couple days ago too, sorry for asking so much about this but as a fresh grad I'm facing a ton of choices at once and could really use some guidance. Coming from a family of restaurant owners, I don't have many people around me who can advise on this kind of path, and Reddit has always felt like the best place for us founders/engineers to turn to. Thanks a lot!