I just went into the sound settings on my two-year-old Pixel, and under “Phone ringtone” there was a selector that was perfectly willing to pop up a file picker

I just went into the sound settings on my two-year-old Pixel, and under “Phone ringtone” there was a selector that was perfectly willing to pop up a file picker and accept a random MP3 file I had lying around. It’s more friction than I think is reasonable for a non-power-user feature, but if you expect the function to be there, you’ll find it by choosing the obvious-looking option at each step.