This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 兘 is uncertain. It visually combines 元 and 台, but the original semantic and phonetic roles of these components in this character are unclear.
Since the meaning is unclear, focus on the shape: 元 (origin) encloses 台 (platform). Imagine an ancient platform at the origin of something, but remember the meaning is uncertain.
For シ, imagine a sheaf of papers on a platform (台) at the origin (元) of a project: sheaf -> シ.