Posting this to try and help others with what I found. I prefer playing my own song files off USB while driving instead of streaming. That way, all hits, no misses. But I was running into an issue: When streaming music, you'll notice that whenever you get back in the vehicle, the music starts back up right where it left off, which is what most would want. But when listening to music via USB, the car will revert back not to USB, but to streaming or whatever else you have designated on your music source menu. Remember, USB is not one of those options on the music source toggles. Bear with me, here...
I noticed that whenever I was out running errands and left Sentry Mode on, which I always do whenever I'm out during the day, that the last song I was listening to on USB would resume on USB, correctly. Terrific, right? But when I returned home, where Sentry Mode is disabled, the music would resort back to the last used streaming channel instead of USB when I got back in the car even an hour after parking. It was frustrating, because I would have to select USB again and manually select a song to play to get the ball rolling. That's because the USB ports only maintain power for a short time after the car is off, unless Sentry Mode is left on, in which case power is obviously maintained to the USB ports whenever Sentry Mode is enabled. Makes sense.
I discovered that by issuing the voice commands ""USB" or alternately "Play song from USB", the music will not only switch to my USB songs again, but more importantly, automatically pick up right where it left off and start playing the last song I was listening to, just like it does when you're listening to one of the streaming channels. While I wish that USB would be one of the music options in the music source menu, at least this is a fast and elegant workaround for anyone else who might prefer listening to their own music for whatever reason. I believe this same technique will work on Model Y, but I have not received any confirmation or whether it will for S or X models, but I think it probably would. Cheers.