How can I set the Microsoft Teams program so that the incoming videos (from other participants) are always turned off by default?
The setting to turn off by default videos for incoming calls is not yet available:
Video is turned on automatically when you join or are called from an unscheduled meeting in a channel. You can't change these video settings, but you can turn off your video once you're in the call if you don't need it.
Source: Turn off automatic video in a call in Teams