flatpak is a method of installing applications in their own containerized environment. Flatpak was develeoped by Red Hat/Fedora, Gnome, and other developers.
flathub flathub.org is a repository for developers to distribute their flatpak applications.
snap is a method of installing applications in their own containerized environment. snap was developed by Ubuntu (Canonical) developers.
That one was renamed. It’s now called Bauh. It has some more capabilities than just managing flatpak stuff.
It aims to be a graphical user interface for managing your Linux applications. It supports AppImage, AUR, Flatpak, Snap and native Web applications.