bright, fiery
爚 centers on intense light and heat: something bright or fiery. The two meanings are closely linked, describing a vivid, blazing quality.
爚 combines 火 (fire) with 龠, which likely contributes the sound and may relate to a kind of ritual vessel or pipe, but the exact historical development is uncertain.
The left side is fire 火, and the right side 龠 looks like a complex container with openings. Imagine a bright, fiery blaze shooting out of an ornate brazier or ritual vessel.
For ヤク, picture a yak standing near a bright, fiery bonfire: yak -> ヤク, and the fire is blazing brightly.