steps leading to the eastern door or throne
阼 refers to the steps on the eastern side of a hall or throne, used in ancient ceremonies. Its meaning is specific to this architectural feature.
阼 combines the mound radical ⻖ (阜, suggesting a raised structure) with 乍, which likely contributes the sound. The character originally denoted the eastern steps of a ceremonial hall.
The left side ⻖ looks like a mound or raised platform, and the right side 乍 resembles a person stepping up. Together they form the eastern steps leading to a throne.
For ソ, imagine a person sewing (ソー) a red carpet on the eastern steps: sew -> ソ, and the steps are being prepared for a ceremony.