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Not sure about “technically” or what the legal situation is. In practice, I know consumer smartphones in the US use a combination of all 4 constellations for positioning. If you have Android, you can easily verify this using various apps. If you're on the west coast (California Bay Area, at least), you can even receive unreliable signals from one or two Japanese QZSS satellites, which are supposed to be regional over the Asia-Oceania region. You can even see a few SBAS satellites (mainly used in aviation.)

My - few years old, mid-range - Android phone shows that it is using 5 of the 6 GLONASS satellites in view as part of its fix. (In California, USA.)

Edit: Using GPS Test: https://play.google.com/store/apps/details?id=com.chartcross...

Have an American model Android phone. Used it in Mexico, the US and Ireland. In all places it used GLONASS, GPS and Galileo equally (assuming equal satellite visibility, of course).