foundation, site, address
阯 centers on the physical base of a structure or location: a foundation, a site, or an address that marks where something is grounded.
阯 combines ⻖ (mound, hill) with 止, which likely contributes the sound and the idea of a stopping place or base. The character is used for a foundation or site, possibly from the image of a mound serving as a base.
The left side ⻖ looks like a mound or hill, and the right side 止 suggests a stopping place. Picture a hill as the foundation where a building is sited, giving it an address.
For シ, imagine a sheet of paper with the address written on it, placed on the foundation: sheet -> シ, marking the site.