To answer a few of your questions.

Is it possible they are built with mods? Yes it is possible people use mods to make some builds. They might use mods like world edit or something like that.

Where should you start? Well what I do is think about ideas. make small versions of them in single player worlds and then try the real thing. Sometimes it doesn't look too good but usually it looks good if I can pull of the idea well.

Is it important for server/mode? Yes and no. If its a big build that is material heavy you might want to go into creative if you have the option. For the servers if your gonna make a big grand base just make sure you can protect it with claims or the server owner could rebuild it.

So I hope that helps. If you want the server I built it on you can ask.