So, now the game is in retail we actually know how it works.
I have the physical CE and inside are the DVD's which you can use to install the game, but not yet play. There is a code inside that you have to use on GOG.com to download the day 1 patch, apply it and play. However, this code you use on GOG.com also gives you a digital copy of the whole game (so not only the patch) and this digital copy is also used for the expansion pass if you choose to buy it and the 16 free DLC they will release.
GOG.com is completely DRM free, so it's just a website with a bunch files (game files, patch files, DLC install files,...) So you can download all and install from your hard drive without ever using your DVD's.
However, GOG.com also has now a Steam-like client called GOG Galaxy. I'm using GOG Galaxy for quite a time now, so I never bothered with the DVD's in my physical CE of The Witcher 3. I entered the code I found in GOG Galaxy, game was in my game library, download, install, patches and DLC and good to go. Very Steam like. I recommend this to everyone with a physical copy unless you are allergic to clients like Steam. But then again, why were you asking for a digital copy anyway?
Cheers!
PS: I just find a bit disappointing CD Projekt Red didn't clearly communicatie about this. I bought an USB DVD-drive just to able to install this game, but afterwards it wasn't needed, because the physical copy does include a code you can use on GOG.com/GOG Galaxy.