How can I paint on a model to create embossed details? I need help to understand what this video is showing.

In this video I want to understand what it's showing when he is drawing a line or some shape on the model surface and later it is converted to an embossed, extruded shape. Does it only work with cylinder shape or any shape? Can anyone please explain to me with an example how it is working?
In the video he is painting to a texture that is then used to control a Displace Modifier which changes the mesh. This method will work on any mesh provided you have a good even UV unwrap and enough face density on your mesh. You will also need to add a subdivision modifier above the displace on the modifier stack to make sure there is enough detail to displace. If your painted strokes do not match the topology of the mesh very well, you may get some jagged edges/artifacts, but these can be solved by retopologising at the end (which you should do anyway).

Towards the end of the video there are subtitles explaining a lot of what he is doing.