In my manual it's not written whether I have to install it into the upper or lower slot
Since you don't plan on adding any other SATA3 drives, you are free to use either M2_1 or M2_2 without any complications on that motherboard. Both slots should work equally well.
I used to have a MSI MPG Z390 Gaming Pro Carbon motherboard myself, and I had two Samsung 970 EVO Plus NVMe drives installed on the board, and I didn't encounter any issues or limitations related to the two drives or which port they were installed in.
As far as I remember, one of the two M2-slots had an integrated heatsink for the NVMe drive, while the other slot didn't.
Just in case your hardware requirements should suddenly change in the future, it could be useful to know, that if you choose the M2_2 slot for your NVMe drive, SATA3 ports 5 & 6 will be disabled automatically.
It is described in the manual, and I'm aware you are not planning on adding drives, but I thought I would be worth mentioning, you never know if it could be useful to know without referring to the manual sometime in the future. If not, no need to pay attention to it
and if I have to enable something before I can use it as boot drive. and if I have to enable something before I can use it as boot drive.
The only thing I remember I would have found useful to know in advance was, that the NVMe drive containing the OS, was not available in BOOT priotity settings until an OS had actually been installed on the drive.
BIOS should recognize the drive regardless, and it should be an available option during Windows install, but I couldn't see the drive in BOOT prioity settings with no OS installed.
I've since learned it is common knowledge, but I wasn't aware of it - so that is the only thing that comes to mind, with regards to BIOS settings. I can't think of any other settinges required to use the NVMe drives on the z390 Gaming Pro Carbon
There is also something confusing written about UEFI and legacy mode, what does that mean?
With the setup you describe, with no dual BOOT requirements for OS'es that doesn't support UEFI mode or things of that nature, I'd rsuggest keeping BIOS in UEFI mode, which I believe is also the actual default setting