I have 2 notebooks with 17 inch screens each. Can the second notebook be connected to the first and used as a second screen? So a laptop and 2 screens?
No. You can connect a normal monitor to a laptop and get a second one. Unfortunately, nothing else is possible.
No, that is not possible.
Yes, this is possible in principle, but it requires tools.
You can e.g. Realize with a capture card. A reasonable capture card usually costs as much as a normal monitor, so it's not really worth it.
A (additional) screen can also be used (at least temporarily) with a SmartTV.