piece together, assemble
凑 centers on bringing separate parts together into a whole: piecing things together or assembling them.
凑 is a simplified variant of 湊, which combines 氵 (water) with 奏 (to offer or perform). The original form likely suggested gathering like water converging, but the modern 冫 (ice) component is a simplification whose historical motivation is uncertain.
The left side 冫 looks like two drops coming together, and the right side 奏 suggests offering or performing. Imagine piecing together a performance by assembling scattered drops into a whole.
For ソウ, picture sewing together pieces of fabric: sew -> ソウ, and each stitch assembles the parts into one piece.