Hi,
SH5 is no longer available to buy on Steam, but it can be bought in Ubisoft Store. You can still play it just fine on Windows 10.
As for installation, I use Steam version so Ubisoft one might slightly differ. That said, you'll have to install Ubisoft Connect after you installed the game (cause old Ubi launcher bundled with the game doesn't work anymore). After that, you should be able to launch vanilla unmodded game but if you want a good experience with SH5, I strongly recommend installing The Wolves Of Steel megamod available here:
https://www.subsim.com/radioroom/sho...d.php?t=210703
It's way more stable, has order of magnitude more content and has been continually updated over the years, fixing most problems with original game. Installation is fairly easy if you follow installation manual to the letter.
TWoS comes with bundled JGSME mod manager, if you want to install any additional mods (make sure they are compatible with TWoS first), you need to copy mod folder insto TWoS_Parts folder in your game and then you can easily enable/disable it with JGSME (the same goes for optional submods of TWoS).