Where the shape comes from
A single dot. The Shuowen explains it twice over — as the flame sitting on a lamp wick, and as the mark a reader puts where a sentence stops. Nobody writes it alone; it heads the small group of characters that begin with a dot: 主, 丸, 义, 为.
汉字 filed under 丶
No published character on HSK Guide is filed under this radical yet. 汉字库.