But where do you find people to form clubs or whatever with? Work/school/office. Also covid wiped a lot of these out and it will take some time for them to re-appear

I have hobbies. Highly recommended!

I had hobbies, now I have kids. But when I did have hobbies where did I meet people to hobby with? Work!

Just for reference, what hobbies do you participate in?

If you can't find people for your hobby and just want to meet others consider volunteering.

> What is left? Literally every other social activity one engages in?

This implies one has energy and resources for social activities. A big chunk of the US is broke and/or exhausted at the end of the week.

The other suggested alternative (religion) generally requires a far higher time + money commitment than many hobbies, yet has thrived for millennia.

For millennia, most people weren't busy sustaining the family during ~all waking hours and year round (all the while living 1 not-rare event away from homelessness).

No, instead most of humanity was living one not rare event away from starvation and/or one not rare event away from death by violence or misadventure.