That's one kind of reaction videos, or like one thing you can get out of react content, but for me when I indulge in it (rarely) it's because it's "X-persona reacts to thing from Y-culture". Mostly I'm amused by Americans watching European things and getting their minds blown. Sometimes it's also because the person reacting is an expert in the topic and provides helpful additional context, for example with fitness stuff it's nice to get a medical opinion about it, etc.
With all that said, you do have to avoid mountains and mountains of garbage content that's only made because it's an easy grift to find the ones worth watching.
They are reacting to the short videos from regretful trump supporters at the beginning. It's even called a reaction in the thumb nail. Thinking about it, what even is the difference between commenting on and reacting to someone else's video, if the commenting is done by showing their videos and then making your own video about it?