to fry in fat or oil, to scald, to seethe, to boil
煠 centers on cooking with intense heat and liquid: frying in oil, scalding, boiling, or seething. The common thread is subjecting something to a hot liquid or fat to cook or process it.
煠 combines 火 (fire) with 枼, which likely contributes the sound. The character is associated with cooking methods involving fire and liquid, but the exact historical development is uncertain.
Fire 火 on the left and the right side 枼 (which looks like a leaf 世 over a tree 木) suggest cooking something in a pot over a fire: frying, boiling, or scalding.
For ヨウ, imagine you are frying food in oil and the hot oil splashes, making you yell 'Yo!' in surprise: Yo -> ヨウ.