Where the shape comes from
A wedge narrowing downward and inward, usually read as the mouth of a dwelling closing in behind whoever goes through it. Set it beside 人 and watch the join: in 入 the short stroke starts at the top and the long one falls away to the right, the exact reverse of the person. It heads 内, 全 and 两.
汉字 filed under 入
No published character on HSK Guide is filed under this radical yet. 汉字库.