Bezieht sich extend nur auf das verlängern/erweitern von Längen ?
Nein, nicht ausschließlich
We have plans to extend our house (= to make it bigger). The government has produced a series of leaflets designed to extend (= increase) public awareness of the dangers of AIDS. We're planning to extend our publishing of children's books (= increase it).
Was ist dann der Unterschied zu expand?
"Extend" is what happens what something long becomes longer. E.g., extend a hand, extend the rod, extend the cord, extend the selfie stick, etc. "Expand" is what happens when a bigger area is acquired or covered. When boundaries are pushed, expansion happens. The sky expands over the ocean. The river expands into a lake. I am expanding my empire.