5e rules don't actually have flanking at all, but some form of house rules for flanking is pretty common. The most common I've seen is that flanking attackers get advantage, which sometimes feels a little too strong. I play in some games where flanking attackers get +1 or +2 on their attack rolls, which feels a lot more reasonable. Advantage is a pretty big deal to give away that easily (not that it's always easy).
It’s a variant rule in the DMs Guide. I think it depends. In TT I wouldn’t give advantage if two guys were standing right next to each other. But opposite sides? I can see that.