Unfortunately the communal bath is pretty much a thing of the past (although I think it still happens in rugby). I had a tour round West Ham United a couple of years ago and the home dressing room was communal shower and a couple of single bath tubs.
As for what they wear underneath their shorts, mostly briefs (bikinis for our American cousins), but compression shorts are becoming more popular. I don’t know whether it’s the players choice or the clubs, because I saw a programme not so long ago about Chelsea FC (W**kers) and they showed their kit man laying out briefs along with the rest of the kit. So I figure that the club must supply them.
I knew some guys who were on an amateur Sunday league team, that for a team building exercise wore thongs for a season. I offered my services as their kitman/towel boy but was not so politely turned down.
What annoys me the most about the beautiful game is that over the past 15 years players shorts have got longer. To the point where when they have their socks pulled up, you only get to see a bit of knee. I remember back in the 70’s and 80’s football shorts were very short and fairly tight. Unfortunately most of the players back then were pig ugly. Now we’ve got some very hot looking fit players in loose baggy shorts that look like the ones they wore in the 1920’s. There’s no justice in the world!!!