There is man flatpak-manifest: Flatpak Builder Command Reference - Flatpak documentation
No, it needs to be installed by flatpak
A remote is what other package managers would call a repository. The term is inherited from ostree.
Assuming you have the flathub remote added to the system installation, you can do flatpak install flathub org.freedesktop.Sdk//23.08, and then flatpak-builder won’t attempt to install it in the user installation.
The correct value for base-version is specific to the base app you’re using. In the case of org.electronjs.Electron2.BaseApp, it corresponds to the runtime version it itself was based on.
See Electron - Flatpak documentation
You don’t need to put that anywhere in the manifest except in the sources section if you’re using a versioned archive or git tag. Flatpak doesn’t know or care about versions; runtime-version and base-version are actually branch names.
You can specify your app’s version in the MetaInfo (aka appdata) file, which is primarily used by the Flathub website and software store frontends.