Exactly what is the process on bringing up an interface in Ubuntu 20.04?

i.e Which process manages this? (Is it systemd-networkd?) And what configuration data does it base this on?

Note: I'm not asking about netplan configuration, netplan is merely an abstraction layer that converts a netplan configuration into a configuration suitable for the specific backend in use - I'm more interested in the underlying functionality of that specific backend, and the process that it goes through.

(Edited to refocus the question to make it clear that I'm interested in what happens underneath the netplan config abstraction layer).