I bought a digital copy of Halo: Reach on my Xbox but can't play it on my PC. Then, I...

The base game is play anywhere, those two are separate purchases

Hi

Halo 5 wasn't available on PC (from memory) so not sure if you will have bought the digital copy from the Microsoft Store for that one.

For minecraft there are a couple of versions. the Java version is only PC but if you bought the digital copy from the microsoft store then it will be available to you on the Xbox.

Forza 3 is available on both and as above, if you bought the digital copy from the microsoft store it will be available to you on the Xbox.

Once signed in on the Xbox (with the same account that owns them) under My Games & Apps they should appear in your ready to install area for you to install.

Is this a digital version of Halo Reach (a.k.a. non-disc) you downloaded on your friend's console? If so, your friend is
probably being blocked from playing it since that digital copy of Halo Reach is registered to your account.

Ultimately, if you two want to play Halo Reach together with just that one digital license, you'll need to play on the same console. I believe, while both of you are on the same console, you both can sign into your own profiles and get Halo Reach achievements
properly (last I might remember, I could be wrong).

If you want to play on your own consoles and own accounts, your friend will need his/her own copy of Halo Reach (disc or digital).

@IIbean: The issues is, I gave away my original copy to someone who didn't have Reach. I expected that this would last me until the next major Halo game came out.

Now since I gave away my original copy, I can't even play reach anymore :/

Yes, it will work on your PC. Halo Wars 2 is an Xbox Play Anywhere title, you will always be able to play it on both your console, and your PC for as long as it is the digital copy, not a physical copy.