copious flow, broad, extensive
瀰 centers on water spreading widely and abundantly: a copious flow that becomes broad and extensive, like a river overflowing its banks.
瀰 combines 氵 (water) with 彌, which likely contributes the sound and the idea of extending or spreading. Together they suggest water that spreads broadly.
Water 氵 plus 彌 (extend, spread) gives the image of water spreading out in a broad, copious flow.
For ビ, imagine a bee buzzing over a vast, overflowing river: bee -> ビ, and the water spreads as far as the bee can fly.
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