shine, sparkle, gleam, twinkle
耀 centers on emitting bright, lively light: shining, sparkling, gleaming, or twinkling. All the given meanings are variations of this same visual brightness.
耀 combines 光 (light) with 翟, which likely contributes the sound and originally referred to a type of pheasant with bright plumage, reinforcing the idea of shining or sparkling.
The left side 光 means light, and the right side 翟 looks like a bird with bright feathers. Imagine a bird whose plumage sparkles and gleams in the sunlight: 耀 is to shine and twinkle.
For ヨウ, picture a yo-yo spinning so fast it sparkles and gleams: yo-yo -> ヨウ, and the spinning light shines brightly.