What's the difference between a diver and an assassin nowadays?

I don't mean this as a literal question, I mean what are the standards that separate the two classes from being the same thing.

Do divers have to be bulkier to not be assassins? usually just go full AP/AD unless they're falling off or behind.

Do assassins have to have instant ability based burst damage? Cause by that description a lot of divers can 100-0 somebody without autos

Do divers have to be staying in fights long, while assassins get in and get out? Cause even the ones who get in, kill something, then get out soak a lot of damage and stick around before they do get out. (would ekko be an assassin for going in, killing something, and pressing r to leave, or would he be a diver for soaking damage with his shield, health, cc'ing something and killing somebody, then reseting? what part of ekko makes him a or b?)

What makes full ad more of a diver than full ad ? They both get in, kill something, and barely get out. They may do it differently, but they still do it.

I'm not asking for changes, but I feel like since mid season 6 where the damage creep for divers is almost the same as assassins, what real niche do divers get that assassins don't, especially when assassins even get the utility of divers with fizz ult or ekko w. I just would like some clarity as to why me one shotting somebody as evelynn or elise doesn't make me an assassin, but part of the diver class.