Do founders outsource design too, or do they just do it themselves?

That really depends on what stage the startup is in. If it's a super early stage, then the founders may do the design by themselves. Seemed daunting, but now there are a lot of AI tools to help us on design.

My boss personally takes care of corporate branding design, but UI/UX is outsourced.

I do it myself as well as most of the tasks. As a startup, it's rare that you have money to outsource

Founders of early-stage startups will try to design themselves and as they grow they hire people for that. If you are looking forward to design, you can try generating designs with Sivi ai.