Hi,

by accident I realized that I can lock the exposure by doubletapping the screen. I don't understand the mechanism that's behind it. The lock icon is square on video and round on photo mode? Is there a tutorial explaining how this feature exactly works?

Don't know if the yellow square/circle LOCKS the exposure.  My understanding is the it will set the exposure for the area you tap on the screen.  Hover and try it above and below the horizon.