2 months old article, but the video explains well what and how Umu launcher works.

Essentially I think it replaces Steam's runtime environment, by doing kinda what this runtime does, but for all games (even non-steam ones). It has a database to match the games, so that umu knows what scripts to run the game (like setting environment variables, installing dependencies etc.).

Lutris now (if I understood well) uses umu when you choose the proton runner. You can enable it in Heroic launcher as well. You can also run umu "bare" through command line, by creating the wine prefix, feeding it a name to match it to the database and let it do its work (I think this is presented at around 01:30:00 mark). As time passes, more programs seem to adopt it.

It's a ~40mins presentation which starts about after the first hour in the video. Took me really long to understand how this works, hope it will help others.😅