usage of the

they are the friends of Tom------->means that all of Tom’s friends are here?
they are friends of Tom------->means just part of Tom’s friends ? right?

“they are the friends of Tom” could also mean that they are some friends who have been mentioned earlier. Your meaning is possible, I guess, but it does not seem a very common thing to say.

“they are friends of Tom” or “they are Tom’s friends” is usual. These do not say anything about how many other friends Tom may or may not have.