[yootyootr]

OP, since you're in the thread:

Did you install a GUI on the Pi?

If not, how does the mpv or ffmpeg command know how to output video to the HDMI port?

[pcouy]

No GUI on the Pi (went with the minimal image).

I don't clearly remember, but I think ffplay detects there is no X server and automatically switches to using Direct Rendering Manager. For mpv, I added a specific option to use DRM output