Where is the buy digital download for this game on XBOX ONE Store?

Ok, then why do i have the option to buy a digital copy of each game, including Gears 1 and 3?

If i go to the store on my xbox one console, i have the option to buy the game.

So i can buy them from the xbox store but i cannot use a redeem code for them.

Also, when inserting the Gears 1 disc, it downloads the digital game on my xbox one as per the console's requirements.

Where does that download come from?

Ok, then why do i have the option to buy a digital copy of each game, including Gears 1 and 3?

If i go to the store on my xbox one console, i have the option to buy the game.

So i can buy them from the xbox store but i cannot use a redeem code for them.

Also, when inserting the Gears 1 disc, it downloads the digital game on my xbox one as per the console's requirements.

Where does that download come from?

If you use the physical disk you are only downloading the emulator patch

Not the game

The digital copy is not yet made backward compatible at the moment although it is expected to be made soon... The disc copy is backward compatible.

Backwards compatibility

According to this link the first Halo in the series is only backwards compatible with disc and not digital

Not sure if it was ever available digitally my guess it was a disc only game for the 360. If you want it on the one you would have to purchase the halo master chief collection.