Maybe on a particular night, but on any longer timescale: how does one make friends who will tell you about the cool underground scene without first meeting them in the aboveground club scene? Maybe online stuff plays this purpose now but I assume its still mostly the former.
lots of places. the people at the warehouse rave usually did something else earlier in the night. maybe you met them at a bar or show and asked what they were doing later. “what are you doing this weekend” is a normal thing to ask anyone you meet in a third place. it’s not that big of a secret.