Where the shape comes from
A cut of meat: the outline is the slab and the two strokes inside are the striations of the muscle. Attached to a character it is written ⺼, which in modern printing is indistinguishable from 月 "moon" — that flesh-月 is why 脸, 脚, 胖 and 肚 all seem to contain a moon that has nothing to do with the sky.
汉字 filed under 肉
No published character on HSK Guide is filed under this radical yet. 汉字库.