flatpaks work like windows standalone executables where each app brings all their dependencies

No thats appimage. Flatpaks run on shared libraries and even different runtimes containing the same packages share those using deduplication

https://gitlab.com/TheEvilSkeleton/flatpak-dedup-checker

A Flatpak is exactly as heavy as a system app, just that on the system you already have some libraries installed.

Initial download size is bigger, okay. And in general more downloads, I guess the deduplication happens on the disk.

Its like, shared runtimes but also not. Its a bad situation tbh.