the xt3 records h265 which is compressed, the atomos records in whatever codec you asked for, so it can be way less thant 74 minutes, or more maybe if you choose the poorest quality
record in either prores or dnxhr
in the recording menu, upper left part of the screen where it says input, click on input and then in the menu go into trigger and set it to "on" and make sure source is set to hdmi