In session 0 we set a quorum. "We play if there are at least 2 of 4 players here," for example.
During a session, if some players don't make it, we'll decide on the spot what to do. This is typically either "They take care of some of their own business" or "we play them by committee". Rarely, it's "the GM plays them."
In a recent game of mine, one of the PCs bailed. His character backstory said he owned a small business, and since the session started in a low tension scene, we said he had to go take care of that.
In a game where we picked up in the middle of a fight, we decided to play the wizard by committee. It was a little slower than normal, but it worked. After the fight was over, they didn't do much other than a few committee approved skill checks. I wouldn't typically make big story decisions or put the character in serious danger when the player was absent.
It's also important to set attendance rules. Are you okay with people showing up whenever? Or do you want to set an expectation that people will be there every week barring emergencies? Those are two different, valid, modes. For a game that's trying to have an arc, and not just monster of the week or a dungeon crawl, having players frequently missing can be disruptive. I typically bring that up in session 0, and say that if someone repeatedly misses sessions it might not be the game for them at this time.