I bought Forza Horizon 3 for XBox One digitally. For the digital version, they offer the opportunity to download the game to a Windows 10 machine as well, given the same Live Id is used for both machines. As a convenient bonus, the game state is synchronized via Live so you can continue your game where you left it on either machine.

The disk version is not bound to any particular Live Id so they do not offer this option for disk-based games.

As a side note, if you like arcade racers, said game is very good entertainment with superb music and feel of speed :)

It only applies to games meant to do so, and so far only those bought through the store. Gears 4 is a good example, buying it from the store gets you a digital copy on your Xbox and your PC -- its the same store transaction, and the same "package", but it might be a different binary/resources underneath for each platform -- no, you can't just put an Xbox disk into a PC and play it -- though its possible you might buy a physical copy for Xbox and get a store code for a digital copy you can play on your PC. With Scorpio having specs capable of flirting with full 4k rendering (and easily able to achieve 4k with tricks like checkerboard rendering), the art assets needed for that will be the same as those on a high-end PC, so its reasonable to think that one day the same exact distribution of bits will power both PC and Xbox machines, even if the actual game binary is different.

Going forward, they've made it clear that convergence is something they're actively working towards, and its something they've been persuing (and productizing) in their OS versions for years now -- Already, every version of Windows, including those on your phone or Xbox One, are essentially the same Kernel and core OS components -- its not like it was even 5-10 years ago where Windows 7, Windows Server, Windows Phone, and Xbox / Xbox 360 all had unique kernels and Core OS facilities (even if they shared lots of code, it might be tweaked or otherwise not a direct replacement). They've converged the store as well -- no more Xbox marketplace, Windows phone store, and Windows store -- its all one store now, with purchases, billing, and licenses all managed the same way.