cook
煑 centers on cooking, specifically by applying heat from below.
煑 combines 者 (a phonetic component) with 火 (fire) to express cooking with heat. The exact historical development is uncertain, but the phono-semantic structure is clear.
The top 者 looks like a person stirring a pot, and the bottom 火 is fire. Picture someone cooking over a fire.
For シャ, imagine a chef cooking and saying 'shah!' as they add a secret ingredient: shah -> シャ.