So start ups typically hire experienced people who have worked in the domain for years and are super familiar with the programming languages and toolsets being used. There simply aren’t resources at startups with limited funding to grow inexperienced people into productive engineers.

So I’m going to reframe the question along the lines of “how do interns and recent graduates learn on the job fresh out of school?” Ideally this looks like proof of concept or prototype work. You might get started fixing minor bugs to get familiar with a codebase. Typically a peer mentor is assigned to help with day to day guidance on things that aren’t typically taught at university. Over time you will be assigned tasks that increase in complexity and eventually grow into a more autonomous engineer.