How can I use the rear facing camera on video conferencing apps like Skype and Zoom?

Zoom has the ability to switch cameras in the iOS app. The switch is available in the top left of the screen once you have started a meeting. It does not appear that you can select the rear camera before you start the meeting; so you will always have a moment where the front camera is "on" before you switch to the rear camera.

If the app itself doesn't support it, you can't do it. It's the code of the app that determines which camera is to be used. There is no system-wide setting to override this.

I don't have Zoom, but Skype does have the option to switch cameras; there's an icon in the top right corner which does this: