Milonga is a place and an event where people meet to dance tango. So the dance floor is the only important requirement.
Other things like chairs and tables to sit at, a bar to order a drink, etc. are optional. Usually it’s there but some times it’s just a dance floor and the music.
The music is tango however there are 3 types of tango music: tango, milonga and waltz. So milonga is both a place to dance tango and a kind of tango music.
People invite each other using “cabeceo” which in simple inviting a partner without words or direct invitation by just using the eye contact.
People dance a set of 3 or 4 pieces of either tango, milonga or waltz and such a set is called “tanda” and after that there is a short musical break of non-tango music which is called “cortina” during which people say thank you to their partner, leave the dance floor, prepare for the next tanda and invite a new partner.