To which input should I insert both an active di-box with a passive guitar and a passive dibox with an active guitar?
MIC input without phantom power (unless the active DI box is powered by phantom power).
To which inputs should I insert all the rest of the guitars without di-box and different types of the pickups?
INST input.
How can my 3-4 inputs be line and mic at the same time, when those are absolutely different input levels?
These are combo connectors, the XLR connector is a mic level input, the TRS connector is a line level input.
What happens to my inputs when I swich on a phantom power? Are they becoming a line inputs so they can handle amplified mic signal?
When you switch on phantom power, the phantom power is activated on all mic inputs. A phantom powered mic does not deliver a line level signal.
As amazing as it may seem, the owner's manual provides useful information.
If I want to record vocals with a powered mic and a guitar through a passive DI Box at the same time, can I do this? Because I have to power the mic inputs, and passive DI is also connected to a mic input.
A decent passive DI Box shouldn't be harmed by the presence of unneeded phantom power on it's output, neither it's sound quality degraded. Nevertheless, I decline any liability about whatever might happen.