We use OBS Studio in conjunction with the teachers preferred videotelephony software (currently Skype) for the piano lessons of our daughter.

I have defined four scenes in OBS which she can switch with A, S, Z and X, followed by a space bar press.

A is the webcam of the notebook, which sits right above the keyboard of our digital piano. This shows just the semi-portrait of the person playing.

Z shows the view from a second webcam mounted above the keyboard to display the keys and the fingers.

X is the view of just the virtual keyboard from a Synthesia window, where the pressed keys are highlighted.

S is all three previously mentioned scenes combined in one nicely arranged view.

Audio from the digital piano is also a source for scenes in OBS.

OBS provides a virtual camera that can be used like a normal camera in most, but not all conferencing software. For example, the preinstalled Skype in Windows 10 doesn't recognize the virtual cam, but Skype Desktop from skype.com does.

It's not perfect, but - I think - a decent setup, quickly built with only a laptop, one extra webcam and free software.