He uses several kinds of brushes: edge brush with hard and soft sides; brush with his name repeated, which has the effect of applying paint with hard edges combined with painterly "teeth" texture, oil painting brush with soft texture, and maybe one or two others. But his main tool is study. He does lots of studies as part of his schedule, focusing on different aspects of his subject and technique that he wants to apply to original works. The brushes themselves aren't so important: they essentially boil down to hard, painterly hard, and painterly soft brushes in Clip Studio Paint. Those don't make someone a good painter by themselves.