how does this works?

so i was watching this lecture by Steve Brunton on YouTube and yk how in his lectures he is writing AT the screen (i’d add a picture for reference but this reddit doesn’t allow it). so i wanted to ask how this writing on the screen works in such videos. i have seen other youtubers do it and i have always wondered if they are writing backwards (only then it look straight to us viewers) or is that they are pretending to write that is just moving the marker and add the text later on at the editing stage? god i don’t know. i would love if someone would tell me how this works.