Thunderbolt iSight Camera Not Working

Is there a way to make the Thunderbolt Display iSight camera the default for all video conferencing? Or is this an app-by-app option?

I connect my MacBook Air (13-inch, Early 2015, running OSX El Capitan 10.11.6) to a Thunderbolt Display that I use as my primary display (my MacBook lid is closed when using). However, I'm not able to use the iSight camera on my Thunderbolt for Facetime, Skype, ZOOM Meetings, etc.