[creatonez]

Ubuntu distributes some system daemons and even the kernel image as Snaps. The Ubuntu Server interactive installer nags you to look at a list of server software (such as nginx) to install using Snap.

Flatpak hasn't really taken the same path, it doesn't have much utility for anything other than desktop apps.

[bonzini]

There is toolbox, see https://github.com/containers/toolbox.

[creatonez]

Toolbox and Distrobox are not based on Flatpak, though. They're more just nicely packaged docker-like container tools, targeted for development use cases.