net
緵 centers on a net, specifically a fishing net or a net-like structure made of thread.
緵 combines the thread radical 糸 with the phonetic component 㚇, suggesting a net made of thread. The exact historical development is uncertain.
The left side 糸 is thread, and the right side 㚇 looks like a tangled mesh. Picture threads woven into a net for catching fish.
For ソウ, imagine a net being sewn together: sew -> ソウ, and the thread forms the net.