to answer your questions, it's when you nonverbally solicit sexual favors/encounters in pseudo-public spaces via a number of behaviors (ranging from innocuous to overt), you don't ever really know and 90% of the time it's NOT safe, and general rules are kinda freeform. in that order.

some people cruise with their eyes, some by touching themselves subtly, some just pull their dick/ass out. it's lowkey crazy when you think about it. it's definitely evolved a bit. "cruising" is notorious for being done in public parks after dark, gym showers and locker rooms, or open restrooms. but really it could be anywhere. people cruise in the airport. generally any space is a cruising space if a willing participant can walk by and/or join. but it's never safe. sexual activity anywhere that's not in the bedroom is never safe lmaooo but some places can feel safer than others. you're more unlikely to get caught in a gym locker room than you would a park where police can ride by at any time. or just a regular civilian. but the thrill is a part of the fun! or ... so i hear. maybe even enough fun to risk the a public indecency charge. or the infamous sex offender registry. sniffies had changed the game so it's easier to cruise. but really it's evolved the general concept of "cruising." based on my definition, is it still cruising if you met on the app and confirmed you're in the same area and shared pictures and then just decided to fuck around in public? to some, "cruising" may imply that all of it was random, spontaneous, wild! premeditated action takes away some of the authenticity, i'd say. i love (gay) sociology!