add, increase, attach, low fence
埤 centers on adding or attaching something to increase or supplement, often in a physical sense. The low fence meaning is a specific application of an added or attached structure.
埤 combines 土 (earth) with 卑, which likely contributes the sound and the idea of low or supplementary. The character suggests adding earth to build up or attach a low structure.
The left side is earth 土, and the right side 卑 suggests something low or humble. Picture adding earth to attach a low fence or increase the height of a mound.
For ヒ, imagine adding a low fence to keep out a he (he -> ヒ) of sheep. The fence is attached to the earth, increasing the enclosure.