The hobbies one for sure is a great one in my opinion but just generally saying yes to invites is the one bit of advice I offer.
When I was 29 I was dumped & miserable, I had 4 close friends who I saw 6ish times a year & I felt so lonely & bored. I had other friends but no-one I saw or spoke to regularly.
I'd been in a dance class for years but one of the women reached out to me when she saw we had a mutual interest in a few podcasts/comedians, she kept inviting me places & I eventually said yes to one event & soon got enveloped into her wonderful friendship group of about 10 people. Of course I'm lucky they were lovely but when I said yes to every invite I could with themI soon weasled my way into this lovely bunch of humans. We go to comedy shows, podcast recordings, weekends away, country walks, have games nights - loads of things.
Around this time I also wanted to try an escape room & none of my close friends wanted to / were free so I went with a not as close friend & we ended up forming a mini team with her, her husband & one of her friends & now I'm super close with all of them as I had a guaranteed escape room outing with them once a month.
I also had a semi close friend & she'd invite me to parties & events where I only knew her in a room of 40 people. Even though I drink I'd always be sober because these parties were in the middle of nowhere so I felt safer driving myself home. The first few times going it was rough but after a little while I slowly got to know all these awesome people. Sure they're not amazingly close friends but they're people I enjoy seeing a few times a year at her events & I've swapped numbers with some & had fun conversations with them about mutual interests & activities. I now really love her events & can't wait for things to open up so I can see these people again.
Honestly when you start realising how much you're actually invited to it makes a huge difference, same for inviting people to do things. If you want to try an activity & you have aquaintances then ask around to see if anyone wants to go. Gigs, mini golf, live comedy, art galleries, sporting events, shows, book signings, conventions - you're bound to find someone who is looking to go to something you're interested in.
It's hard at first but once you get into the swing of things it's great.