Hi,
The fake_extension is used only in unit tests to test the extension framework, i.e. :
If you would like to write an API extension check neutron-lib/api/definitions/ (and you can find the extensions "counterpart" under neutron/extensions in neutron repository)
You can also check other Networking projects like networking-bgvpn, neutron-dynamic-routing to have examples of API extensions.
If you have an extension under neutron/extensions and there's somebody who uses it (see [1]) you will see it is loaded in neutron servers logs (something like this: "Loaded extension: address-group") and you can find it in the output of openstack extension list --network
Best wishes
Lajos Katona