Chinese normally come up with some ways to use old terms to describe new things.
First character means "net" (can also be fishing or spider net)
Second means "line, route" (also way, sequence)
Third means, well, "hole" (but also socket)
As a combination of first and second we have "net line" - the Internet, or really any interconnected net like a power grid.
Third doesn't need an explanation, I think.
Do they need to make a new symbol every time something new is invented?
I mean it's exactly the same as inventing a new word whenever something new is invented. It's usually derived from already existing words and so are the symbols.
Same way as English, by combining preexisting words. For example, ether + net + port = ethernet port. In this case the first two characters (that's what they're called) mean "net" and "road", combining to mean "network".
I think adding new characters is very, very rare. Would be kinda like adding a new letter to the alphabet.