Where the shape comes from
A wide, stemmed dish seen from the side, base and rim both visible — the generic vessel that sits beneath a whole family of characters for containers and what they hold. Water shown overflowing its rim gives 益, to overflow, and by extension benefit, gain; a lid fitted over one gives 盖, a cover, a lid; and it simply names the tray or plate itself in 盘.
汉字 filed under 皿
No published character on HSK Guide is filed under this radical yet. 汉字库.