The fans are already installed. You only need to plug the cables into the mainboard - provided there are enough fan ports on the mainboard. Otherwise, order a Y-splitter at the same time...(see my review).
RJ45 ports are usually not on the housing, but on the connection side of the mainboard (at the back).

Yes and yes, except that the fans are not RGB (led).

I don't understand the question about the Ethernet, it depends on the mainboard.
There are 4 case fans included, but without lighting

Enthernet (RJ45) is plugged into the mainboard and not into the housing. The choice of mainboard therefore depends on what you install there.
ATX is recommended for the size of the mainboard. As in the description, 4 fans are already installed, but only standard fans without RGB and therefore without lighting.

Hi, it depends on what kind of mainboard you are using. As for the fans, there are already 4 of them. Simply without rgb if they are important to you.