Samsung is now releasing a new update for Bixby Key. This new update enables the user to remap their Bixby Key to open any app they want. The new update finally keeps the promise that Samsung made during the S10 event.
You cannot disable Bixby completely, obviously. However, you can add new apps when you either double or single tap the button.
After you enable the option, it looks like this :
Once you select either of the given options, you are then allowed to set the app as you wish.
After you select your favorite app, for us WhatsApp:
Well, you cannot switch to Google Assistant anyhow, obviously.
Source: Android Central