stone base, plinth of a pillar
磉 centers on a foundational stone element in architecture: the stone base or plinth that supports a pillar.
磉 combines 石 (stone) with 桑, which likely contributes the sound. The character refers to a stone base or plinth.
The left side 石 is stone, and the right side 桑 looks like a mulberry tree with branches and leaves. Imagine a stone base carved like a sturdy mulberry trunk supporting a pillar.
For ソウ, picture a stone base so strong it can support a whole soul: soul -> ソウ, and the stone plinth holds the pillar's spirit.