pure gold
鎏 centers on the concept of pure gold, specifically referring to fine, unalloyed gold of high quality.
鎏 combines 流 (flow) and 金 (metal/gold), likely suggesting gold that has been refined or flowed into a pure state. The exact historical development is uncertain.
Picture molten gold flowing (流) and settling into a pure, solid form (金). The flowing gold becomes pure gold.
For リュウ, imagine a pure gold statue of a dragon (竜, りゅう) being cast from flowing gold: 竜 -> リュウ, and the flowing gold becomes the pure gold dragon.