No one can really answer this question for you since every person (and every medical school) is so different.
I was not that close to friends in high school (I don't keep in touch with a single one... kinda said "good riddance" when I left high school). I still talk to a few undergrad friends who I was pretty close with. That being said, my closest friends by far were made during medical school, and I know that's the case for many in my med school class outside of my friend group as well. I never went to bars or clubs until medical school, but I went out to them a couple times a month during med school, especially during the first two years where everyone goes out to celebrate together post test. My med school friends were there for me during multiple family tragedies while I was far from home, and I knew they always had my back. We would study together in the library or coffee shops, work out together, go on group camping trips or post-board trips, celebrate birthdays together, and just enjoy each other's company (game nights, movies, sleepovers, museums, the zoo, hockey/baseball/football games, karaoke, scavenger hunts, dinners, cooking/baking together, etc.). Now, we're all spread across the US, but we keep a group chat going and keep in touch other ways. We still crash at each other's places if we visit their cities, and several of us edited each other's personal statements for fellowship. When two of my med school friends just married each other, we all flew in from all over the place to go to the wedding and took some of our vacation time to take a road trip together for a few days.
Of course I'm not encouraging you not to have friends now.... everything is easier with support. However, despite med school being busy, you might find that you have more time than you think. I think it was easier to find good friends during med school because you meet people with similar interests, you're all on the same schedule for the most part in preclinical years, and your only responsibility is really school/studying and not trying to split time between work, multiple volunteering gigs, research, clubs, etc. like high school and undergrad.