shine, flourishing
曄 centers on radiant light and vigorous growth: shining brightly and flourishing with life.
曄 combines 日 (sun) with 華 (flower, splendor), suggesting sunlight that makes things flourish. The exact historical development is uncertain, but the component relationship is clear.
The sun 日 shines on a splendid flower 華, making it flourish and glow with light.
For ヨウ, imagine the sun shining on a flourishing field of yarrow (a plant): yarrow -> ヨウ, and the light makes everything shine.