I installed it today and it works! Very simple! Thanks to everyone who answered!

We don't have internet-box 2, but Standard (Swisscom). In my opinion, you can connect via LAN cable without any problems.

Basically you can, yes. However, depending on how you want to use the Decos, you may have to switch off DHCP on the Internet Box and set up the Internet connection on the Deco accordingly. Alternatively, you can configure the Decos as repeaters, which is easier but in my opinion a waste of money. You'd be better off buying cheaper repeaters.

Yes, but you still have to configure it....

Yes, it is possible to connect it. There are videos on YouTube that explain how it works.
I'd recommend them if you need to set up a complex wifi environment with more than 20 devices under management. I find them expensive compared to what others do, but they do the job well.