I have a video of a friend playing a simple tune on their piano. The video shows the right hand movement and I can play that part myself, but the left hand is off shot. How can I work out the chords being played? Is there a service for that?

I'm sure there may well be a techie solution these days but in essence, the melody (or tune) you hear will fit with certain chords and for me, it's just trial and error, listening for what sounds 'right' (to my western musical sensibilities) as I play the tune and try out root notes, then more complete chords.

ChatGPT tells me:

Band-in-a-Box can do this. You record your melody from a MIDI keyboard, then its Reharmonist suggests a chord progression in your chosen style, showing chord names such as C, Am, F and G. pgmusic.com