large tripod cauldron with small mouth
鼒 refers to a specific type of ancient Chinese ritual vessel: a large tripod cauldron with a small mouth.
鼒 combines 鼎 (tripod cauldron) with 才 on top, which likely served as a phonetic component. The character specifically denotes a large tripod with a small mouth.
The bottom 鼎 is a tripod cauldron, and the top 才 suggests talent or material. Imagine a talented craftsman making a special large cauldron with a small mouth.
For シ, picture a sheaf of grain being cooked inside the large tripod cauldron: sheaf -> シ, and the cauldron's small mouth keeps the steam in.