I've got to believe that just having the Subaru Solterra Connect app running in the background will be enough for it to work. (If not, I'm going to be really mad at that lady in the video for misleading me!) I installed the Toyota app while waiting for a RAV4 Prime and the only way to close it and stop it from searching for a car is to Force Stop (on Android). Subaru Solterra Connect seems to behave the same way.
I think the use case for the Digital Key is very limited, the way that it's been implemented. I suppose I could leave the fob at home when I'm going to work once I know that I have a good signal for both the car and my phone there. Then, I just have to hope that Toyota's servers don't crash!
It could be nice to disable the fob and throw it in the car and use the phone during outdoor activities where you don't want to have to carry both. At least then, you'd know that you have a good signal when you lock the car with your phone. Sharing the Digital Key with others sounds nice, but will there ever be a time when it's not just as easy to hand the fob to someone? And with the Toyota app as buggy as it is (it's getting better), do I really want to make someone install it just to use my car?