I have an Xbox one in my home. If I got a PC now, could I then load purchased games like Sims4 and Minecraft onto the PC without paying any more money?
You open the "Xbox App" in Win10 and log in (enter the key of the Xbox CD, if necessary) and play on the PC, but you can't change the graphics, because it's just an Xbox game.
Or that's in the Windows Store
This is conditional! Windows 10 with the Windowsstore / XB Store are limited crossplatform compatible. At Sims4 / Minecraft I'm sure that it will not work.
Sims4 on PC runs over Origin.
Nope - but yes… But not legal…
But:
You should be able to play certain Xbox Live titles on both platforms if you log in with the appropriate login - but I have little experience with them (yet), as I hardly play such titles and the fun is still relatively fresh.