All you can hope is that some day they are forced to support 3rd party apps because of some anti monopoly lawsuit telling them they have too.
I’m not sure if you are aware already but the reason Epic are announcing a million changes to their business is that the previous business plan was based around them successfully throwing a fortune at suing Apple to force them to support 3rd party apps and they tried and failed.
I think if it were ever going to happen that way, Epic would have succeeded.
That’s not to say it won’t still happen one day through political means. Seems plausible the EU might force it at some stage.
AFAIK it’s a system to let Linux software bundle all of it’s dependencies up with it so it just works in a self contained way that doesn’t care about what else is and isn’t installed.
Advantages is that they are more reliable and user friendly than traditional approaches to Linux software installation.
Disadvantages are that they have bigger footprints where you might have the same dependencies I dependently installed for each app rather than as a single installation that they all utilise and that they need to be updated individually (as part of the flatpak.) IE if basically every app uses the same dependency and it turns out to have a huge security hole, under normal Linux software the developer would patch it, you’d update it and the hole would be filled. With Flatpaks you need each individual Flatpak developer to update the version used by their Flatpak and for you to update all those Flatpaks before the hole is plugged. I think I remember they run in some kind of sandbox to mitigate this though.