https://www.youtube.com/watch?v=YuVJ7lP5Sfc&t
At the beginning of the video you can see that he adds a lot meshes.
How does he do that so fast? Because when I add two objects and then press Ctrl+J and try to sculpt, they do appear as one object but I still got some sculpting bugs (they cant fully "Connect").
In another section of the video (0:10), he somehow paints exactly on the mesh and turns it into a 3D object (the shoulder armor) and then adds a cylinder there.
Similar examples (1:48)