Where the shape comes from
A mortar bowl seen from above, hollowed out with grain scattered inside it, ready for the pestle. A hand reaching claw-like into it to scoop something out gives 舀, to ladle, to scoop; a pestle raised above it gives 舂, to pound grain, the mortar shown doing the one job it was carved for.
汉字 filed under 臼
No published character on HSK Guide is filed under this radical yet. 汉字库.