MTG Arena - casting during opponent's upkeep
There're two ways to do this. The first way is to hold full control. The default keys to do this is Ctrl + Shift, and it'll keep full control on until you turn it off.

The other way is to set a stop on your opponent's upkeep. You do this by clicking on the icons under their avatar (not the ones just above the "pass" button on the bottom right), and the game should auto-pause on their upkeep and draw steps.