Plugin integration

Good day! How exactly does a plugin of a service link up to the main
service? I looked in the C tutorial's subsection regarding DHT and the need
to provide a block plugin in order to validate requests/replies and it's
not quite clear to me how exactly does the service use it in its inner
processes. On a sidenote, would it be an issue to use the same block
validation logic in the FS block plugin?
Another question is about the DHTU, does it also work as a plugin and would
it allow me to customize the underlay in order to guide each message in the
network?

Thank you in advance!
Andrei Usurelu