The 匕 radical at the lower-left is the semantic core; it originally pictured a small ladle or scoop. 匙 effectively writes 'spoon' with 是 as a sound clue. The same radical anchors 比 and 北.
Top-left bulk supplies the sound, shì drifting heavily to chí here, one of the larger phonetic gaps in modern Chinese. 是 sits in the dominant upper position; 匕 wraps around the bottom-left in the surround pattern.
