Beginner questions

I'm just confused about when you scroll around with the camera over a planet and come back to your home base, it's likely that the viewpoint is completely different than before. I will try the 'n' trick, using this view right from the beginning for orientation.

Yeah, the camera anchor points that @mwreynolds mentioned are a good help for that too. I'd like to add to that that the first camera anchor point is already set automatically. So <Alt>+1 will always bring you back to your main base with the same orientation and zoom level as it was in the beginning. (Well, assuming you built your main base where your comm landed).

I think this is just something you have to get used to. Seeing up camera anchors as others have said definitely helps. Also if you want to get the camera in a specific rotation before you set a camera anchor you can kind of scroll in a U shape (down, then across as much as you want and then back up to the original spot).

Ok.

But what I'd really like to know is this different circle for certain buildings. Also the 'anti-nuke launcher' has three circles.

Did not know this. Good one.

No one can explain the different colored circles of multiple buildings???

I think green is radar vision, red is ground/air attack range and orange is orbital attack range.

Next question:
I can build an advancded metal extractor over an existing normal one. Is that a bug? Which income does count? Both?

Both

It's intentional, if you zoom in really close you'll see the t2 is designed to fit around / over the t1