How to convert a certain program's audio output to mic input

Solution 1:

You can use something like Virtual Audio Cable.

There you can set different Audio Devices and reroute them how you like it. You could for example output Spotify on Cable 1 which is also used as an input in Zoom. But your browser for example uses Cable 2 which is used to route the output normally to your headphones.

There is a free version where you get an ad every now and then to test things out, but it's quite handy for specific use cases.

EDIT: It seems they changed their license model to a different one where most of program is free, but you'll have to donate to use more inputs or outputs.