Community gardens, sports, religious or interest groups, collectives, contributing in a large household, early work experience, hobbies/interests. It probably is a fairly finite list because societies have optimized for the individual and people are often only active within of a community at work or in education facilities. So a part of a solution in my view would be establishing more communities that are separate from the family... they might look a lot like schools though, so maybe we should just focus on those? There's more need for new communities to be established for other age ranges.
I pretty much agree 100%. We need more, smaller communities – and we need them offline.
Notably, this is largely an American problem, since America is built around cars, which given the capitalist nature of American society proves to be antithetical to establishing local communities.
European and other countries, whose layouts and culture were established in pedestrian days, are much better off in this regard.