After years of avoiding open mics like the very plague I now go to 3 on a semi regular basis.
Open mic in a pub - sign up sheets, collabs, solo performers, improv jam session, some times a bit over run with Ukuleles, but other times steel pan, and other odd instrument players arrive to change up the vibes, it is generally great.
Afrobeat Jam in a record shop - House band invite players to join them for improv jams, no sign up sheet, no Ukuleles yet. Got to have some chops to keep up with the HB but welcoming and good vibes, musically interesting.
Jazz Jam in a pub - House band plays then just leaves, cue desperate musos fighting each other for a spot then proceed to make a collection of independent "jazz" solos dominated by the drum solo, more of a sonic jumble sale than a jam imho, I might go back when I grow a new ego. Disorganisation aside it was ok perhaps I arrived on a full moon.