You can picture inset as making a space in the surface of something and putting a smaller thing down into that space, so the small thing ends up level with the surface and counts as part of it. Nothing is stuck on top and nothing hangs off; the small thing is held inside. Once you can see that, the word works the same way whether the surface is a wooden lid, a ceiling, a ring, or the printed frame of a map — and it names both the act of putting the small thing in and the small thing itself.
From the word’s history
The supplied Wiktionary entry traces inset to Middle English insetten, from Old English insettan 'to set in', which is simply in- plus set. The modern word still says exactly that: something is set in. This is a historical note, not a guess made from the picture above.
packet 裡的 Wiktionary 條目寫著：inset 來自中古英語 insetten，再往前是古英語 insettan（set in），也就是 in-（進去）加上 set（放置）。今天這個字說的還是同一件事——把東西放進去。這是來源說明，不是從上面那個畫面倒推出來的。
Wiktionary entry for “inset” supplied in this packet (kaikki extract, source_ref kaikki:3f6f69a190036ae4:184446988): “From Middle English insetten, from Old English insettan (‘to set in’), equivalent to in- + set.”
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