卅 is a Chinese hanzi, listed here under the keyword “thirty”. It is built from 川 (stream) and 一 (one). It appears as a component in 1 other character in this index.
On KumiJi Glyph Explorer, open the parts search and type “stream” + “one”. 卅 is among the results, together with every other character containing the same shapes.
Open 卅 in the interactive kanji search
to expand every part down to its smallest pieces, add your own names and mnemonic stories,
or search for other characters by their parts.