It's more than a packaging format but yes. It includes all dependencies needed to run the main program in a container but the kernel. It's a complete separate root filesystem. When you run it, as intended, a single process is started which loads all the things it needs from that filesystem. It's isolated from the rest of the system unless you share resources with it, like directories or special devices. Obviously this results in larger packages but there is a clever way to save on that overhead with layering, so in practice while still significantly larger than single program deb files, it's not nearly as bad as it sounds. The thing is that Flatpak and Snap also package dependencies to a different degree.

You can think of Docker and Podman as an almost zero overhead (CPU and RAM) way of running one distribution on another. So, you can run an application in Docker that expects to be running on a different distro from what you use (say Ubuntu Jenkins but actually running on Debian). The environment that the applications run in are called “containers”. Mostly they contain the filesystem layout and application libraries that the app expects.

Docker itself is designed to sandbox the application away from your host system. A related technology, Distrobox, uses the same containers but in a way that the applications know they are running on your system with full access to your display manager and home directory.

I run an Arch Distrobox on every distro that I use. This allows me full access to all the Arch repos and the AUR even on other distros ( eg. Alpine, Chinese Linux, or Debian).

Flatpak also uses containers and so you can consider Distrobox as a Flatpak alternative. Flatpak containers are not the same as those that Docker uses but they rely on the same underlying Linux kernel features to do what they do. In Flatpak, you are essentially running the Freedesktop distro on top of your host distro (so much like Distrobox with the guest distro chosen for you).