I'm on NixOS right now (wanted to learn it as a project), but was on bazzite before and it's the first distro we're I no longer felt the need to distro hop. I even installed it on my friends computer who doesn't game, just because it's the closest I've experienced to "it just works" out of the box and stays working. It's what I've been recommending to all my ex-windows friends. Bazzite sounds perfect for you then.

So the OS can rollback (and if a boot fails it will automatically try a previous "image" to try and boot successfully). But your user "land" isn't controlled by the OS, so it's always a good idea. Especially because that's where all your files and configs are. You never know if a hard drive will fail or fry, and having it backed up will save you from that panic. If you do setup a backup solution (whole other conversation) you only have to care about your /home directory, and some people might also backup /etc. for software, besides the one they recommended, "sync thing" is good for syncing files between devices and "borg backup" is good for doing more traditional backups.

TLDR: automatically, but you can manually set things.

Every time the OS upgrades. It keeps an image/version/generation or 2 automatically, but you can also "pin" a working generation if you want to to not eventually get blown away (it's a rolling couple versions, so it will delete older ones and it updates and adds new ones to save you space). The updates are manual unless you set them to be automatic.

Flatpaks are "containerized", but you don't have to learn docker or anything. "Bazaar" is their app store for flatpaks. Basically when you want to install software, open it up, search for what you want. And click install. It handles everything else for you, including updates for the flatpaks. Basically you don't even have to know their flatpaks. Just know to use the app store "Bazaar" instead of googling for Debs or using apk install.

Distrobox is a container "toolkit". But honestly the GUI tools bazzite provides pre-installed abstract any knowledge you would need to learn away, you don't have to learn or understand containers, all you need to know is "if I can't find it in Bazaar, ill have to install it in distrobox". Then you use the GUI tools (sorry, keep blanking on the new one they have is called, maybe "Pods", old one was "BoxBuddy"), create ONE distrobox container (for you probably select Ubuntu or Debian from the drop down), leave the defaults. And then run any "apk install" commands in it instead of bazzite.