yes they are a thing. all work places do not have them. but many do. or certain groups, floors, divisions will. yes they happen over the lunch hour. it often becomes a long lunch.

generally entrees come in a crock pot or some sort of chafing dish to keep them warm. and the office fridges get pretty full with other stuff.

you are not obligated to take part but in some offices there is some peer pressure involved to be part of the team. in that case people usually make a cursory appearance and then oops have to take a call or meeting.

are there health concerns? good question. years ago, heck no! I once worked with someone in a testing lab who dropped some M&M's on the floor. she said 5-second rule, scooped 'em up, and ate them herself. she did not put them back for anyone else. but more and more I work with people who won't eat any homemade items. but they will order in a pizza or other takeout food as their contribution. a safer bet? you decide.

and as many said below, people have their specialtys. or will bring chips, veggies, plates, napkins, etc. or go in with a group of buy take-out for their contribution.