How can i find extensions for a runtime?

So, since the GNOME runtime builds upon the Freedesktop one, it uses its SDK extension for this purpose.

All Freedesktop SDK extensions follow the id schema org.freedesktop.Sdk.Extension.EXTENSION_NAME.

For a basic list of all extensions, you can use Flatpaks search:

$ flatpak search org.freedesktop.Sdk.extension

Most, if not all, of the extension manifests are in the Flathub repository. Some of them have a ReadMe with additional information, but not all.

Here is a link listing all SDK extension repositories:

GitHub

Flathub

Flathub is a build and distribution service for Flatpak applications. Its goal is to act as a central hub for making desktop applications available to users. - Flathub

For Rust and Node specifically: The Rust extension is org.freedesktop.Sdk.Extension.rust-stable. For Node, there are different extensions for different Node versions, for example org.freedesktop.Sdk.Extension.node24 and org.freedesktop.Sdk.Extension.node22.