THE Photoshop Brush!
We get asked that question from time to time. The "pencil" brush that Aaron uses in most of his videos is in this set https://creatureartteacher.com/product/aarons-custom-photoshop-brushes-set-1/
In this image it's texture example #7
In photoshop when you load the brushes by default it is the 7th one in the list (highlighted in orange box) below.