I didn't know what to get my friend tingting for her birthday, but in the end I got her a ring.

at the end: "at the end of something" ; is generally followed by "of something".

in the end: it is a fixed expression that just means finnally or eventually. It doesn't tell us at the end of anything specific.

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At the end of the lesson, I talked to my friends about going to a restaurant, and in the end, we decided to go to a Japanese restaurant.

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There is also a fixed expression "at the end of the day".

It is often used by natives to mean "in the end", not literally "at the end of a specific day".

I didn't know where to go to for my holiday, but at the end of the day, I decided to go to Japan.