Where the shape comes from
A rock ledge jutting out over the ground: the short horizontal is the overhang and the long sweeping stroke the face of the cliff dropping away. People sheltered under such ledges, so it heads 厅 and 厨, and the stone lying beneath one is 石. Simplified writing then gave the bare shape to 廠, which is why 工厂 is now written with two strokes.
汉字 filed under 厂
No published character on HSK Guide is filed under this radical yet. 汉字库.