Inlay can be pictured as making room and then filling it exactly. A space is cut into a surface, a piece of some other material is shaped to that space, and it goes in until the two are level. Nothing stands up above the surface; run your hand across and you feel one thing, not two. That is the pattern on a wooden box, and it is also what a dentist does to a tooth: clean out the hollow, make a piece that matches it, set it in.
From the word’s history
The source analyses inlay as in- + lay, literally 'to lay in'. The modern meaning has not moved far from that: the material goes into the surface rather than onto it.
字典把 inlay 拆成 in-（進去）加 lay（放、鋪），字面就是「放進去」。今天的意思離那個字面沒有走遠：材料是進到表面裡，而不是留在表面上。
Wiktionary (kaikki extract kaikki:3f6f69a190036ae4:1253461737:9430), inlay: 'From in- + lay.'
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