Picture whatever it is that takes the earth out of the ground and leaves a hole behind. The word does not say whether that thing is alive; it only says what it does. English builds words like this with -or, an ending that covers both a person who does the job and a machine that does it, the way actor is a person and elevator is a machine. So excavator points at the digging, and the sentence around it tells you whether a yellow machine or a person with a brush is doing it.
可以想成：那個把土從地裡弄出來、留下一個洞的東西。這個字並沒有說它是不是活的，只說了它在做什麼。英文用 -or 造字時，這個字尾同時能指「做這件事的人」和「做這件事的機器」——actor 是人，elevator 是機器。所以 excavator 指的是「挖」這件事本身，至於挖的是一台黃色的怪手，還是一個拿著刷子的人，要靠句子裡其他的字來判斷。
From the word’s history
From excavate plus -or. The stem names the action of digging something out, and the ending names whoever or whatever performs it.
來自 excavate（挖開）加 -or。字根講的是「把東西挖出來」這個動作，字尾講的是「執行這個動作的那一位或那一台」——所以同一個字自然涵蓋人和機器。
Wiktionary, "excavator"
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