In bunpro, it says that one of the meanings of ～てしまう is to finish completely and that verb + 終わる means to finish/to end. What is the difference between these two since it seems like the same thing? Or is one more common than the other?
Examples sentences from a dictionary of basic Japanese grammar:
昨日その本を読み終わった/読んでしまった。
今朝九時にやっとレポートを書き終わった/書いてしまった。
If they are interchangeable,
could てしまう be used in this sentence: 荷物は全部届け終わりました
And 終わる in this sentence: 友達は私のケーキを食べてしまった。
In the sense of completion of action, both are to some extent interchangeable, but 終わる is just neutral to finish while てしまう adds some nuance. So ultimately it depends on the context whether or not one can be replaced by the other.
Standardly てしまう has two meanings: 1. 完了 and 2. 後悔・残念 (e.g., 1, 2). As mentioned in the first link, V+てしまう can be ambiguous (宿題をもうやってしまいました can be 後悔).
That said, 友達は私のケーキを食べてしまった would be felt as 残念 (to the speaker), so 終わる cannot replace it here. For the other three examples mentioned, both 終わる/てしまう can be used, but to me, using てしまう sometimes adds the feeling of 'having done away with (often something nasty)'
Note that the added nuance is a matter of context (or imaginary common situation where the sentence is used). 荷物は全部届けてしまいました could be a simple reporting of completing delivery. But レポートを書いてしまった may often imply reluctance of writing the essay on the subject's side.
