Anecdotally here's what's worked for me:
1. Intramural sports- even if you're not sporty, some leagues have zero barrier to entry (Bowling or sand volleyball 6's). There are usually pickup sports apps as well that you could probably find based on your city (i.e. JustPlay)
2. Volunteering- especially neighborhood / trail cleanup groups that you can find on Instagram or Meetup. These are low commitment and people tend to be friendly. Habitat for Humanity was another one that had lots of young people new to the city.
3. Free Fitness (or any other hobby) Groups- you can find anywhere. November Project, for example is in most cities
Otherwise, previous friends knew people and put me in touch. And finally there were 1-2 people who had also moved to the same city at some point, making it easy to plug into their networks
It was frustrating and discouraging for a long time and then suddenly I found myself with people to call on and spend time with. Half a decade later I moved and had the opportunity to try the experiment again. This time secondary connections made finding my network almost trivial. Maybe age helped too, being further removed from the university cliques.
I was relieved to find out, albeit way after the fact, that many have this experience. One friend recently quit his job to start a company focused on this phenomenon [0]. I hope he can displace the zombie of Meetup.
[0] https://getopenmat.com/