Bucha was a town near Kiev that was taken by Russia and later retaken by Ukraine after Russia retreated. Ukraine brought in the Special Forces Regiment Safari to clear the town of collaborators, saboteurs, and Russian accomplices. Some Azov guys were there too for some reason. Several days after Ukraine began their clearing operations they found a large number of dead civilians that they blamed on Russia. The bodies were wearing civilian clothes and had white armbands.

There was a video later released of Russian paratroopers executing about 7 disarmed troops in Bucha. They checked them for tattoos and released at least one for only being in the TDF (state militia). The guy that was released was accused of treason by Ukraine, I don't know what happened with that investigation.

Basically everything else is up for debate. Good luck trying to figure out what actually happened. Occupying soldiers do love to do warcrimes and the more elite they are the more they love warcrimes. Ukraine has been declaring people as traitors and collaborators for things like accepting Russian humanitarian aid and teaching to the Russian curriculum plus their far-right paramilitaries aren't too concerned about due process.