Working on own startup instead of an internship. How to make sure I'm learning?

I'm a junior in college majoring in computer science. This summer I decided to work on my startup instead of getting an internship. But I see all of my friends spending 8 hours a day and learning how to work in large teams and learning new technologies. How do I make sure that there's no stagnation in what I learn or is this something to even be concerned about? Would love your feedback/personal experience! Thanks (In my startup I'll be working mostly on iOS and React for dev stuff)