I can't stress enough how important it is to have a session zero to come up with a group contract. It's become customary in groups i play in and it saves so much headache.

This goes for organizational things like start times - both of my current online groups include several parents, so we've planned for some wiggle room in regard to start times. We've also gone into things like "microphone discipline" (not talking over others), which is a huge deal in one group because we're six players + GM and bringing it up beforehand has made things a lot more manageable. We've also agreed to set up chat channels where people can just be silly and shoot the shit so it doesn't distract the entire group. We're not exactly running a serious game there, but it's very heavy on drama and character play, so it helps when the beer and pretzels type stuff is a bit seperate.

We've had spread sheets with potential lines and veils (CW-type stuff that either doesn't appear in the game at all or only happens off-screen) so people can make clear in advance that they don't want certain topics like SA, torture, gore, are phobic to a specific type of animal, do not want sex and romance or sillyness in the game or absolutely do want that etc. The spreadsheet format means that nobody has to open up to the entire group about trauma if they don't want to. ofc there are some issues we discuss more openly if need be, for example i'm not the only trans person in both of my groups and the GM in one campaign made sure to see where our boundaries are because the setting is a highly gendered society that also has non-traditional gender roles. Now, you may argue that certain things like not wanting assault and bigottry in your game are self evident, but unfortunately they're not. Having a CW spreadsheet reliably scares off the creepy fedora guys, and i've found that there's always at least one issue that's problematic for somebody that nobody would have expected.

Another huge aspect of this are expectations towards the game mechanics and how to apply them - like how to handle character deaths, if fudging dice rolls is ok or not etc. Resurrection is extremely rare in our setting, so most players don't want a character they're attached to die due to one unlucky dice roll. OTOH we have a character who's a cleric of a duelling goddess with the biggest "bring it on" energy possible and his player's been very open about having no problem with a heroic perma-death of his character. Knowing that in advance instead of going on assumptions what's "normal" and "expected" and "common sense" avoids a huge amount of arguing. Same goes for things like how much to minmax and balance characters, balance between roleplay and rollplay etc. How to handle arguments about rule interpretations should also be a part of this.

Then there's the characters themselves and their interactions. Do any of the PCs know each other from before the campaign? What are their motivations, hopes and fears? Is there certain content that would be cool for the characters to deal with? How does the group want them to interact? Do any of the characters havbe longterm goals and can they become a part of the campaign? Have they already made enemies that could come up as a nemesis? How do we handle that? We also encourage to frequently have breakout sessions were people can have 1:1s in a play by post format. Thanks to this, my bard is now in an extremely cute relationship with the group mage and has a growing rivalry with the nepo baby elven prince who's also in our group. In situations like that, it's important that everybody is on the same page about how to handle drama between the characters, how much PvP is permissibly, whether any lewd stuff is allowed to happen between PCs and how (only in DMs, only in the chat in spoiler tags, etc.) and things like that. OFC if this goes into ERP territory, all players involved should also set boundaries in advance, not to mention that this isn't something that works with every group or every player.

Play styles are so vastly different. You really can't take anything for granted. The best way to deal with this is to take the time and talk about it before the campaign starts.