How to play one-channel audio in both headphones?
Two of the comments made by @AndrewT have suggested to me the good solution:

true mono audio shouldn't be a problem when played on stereo speaker
(it will play the same waveform on both left &amp; right channel)

and

the audio source is actually stereo, but the audio is only recorded on
1 channel

As the youtube audio in question is not a mono but a stereo with just one channel (Audacity was right, Mediainfo was wrong), a solution is exactly the one that initially seemed to me the opposite of what I wanted: rendering the file as mono — and thus, playing it through both headphones.
This solution works on Android 10, the setting is under Accessibility, Audio&amp; on-screen text.