At the end means the time or point that something ends. The grammar is at the end + of + noun:
- At the end of the day, I like to relax while listening to some music.
- At the end of the movie, the hero defeated the bad guys.
- You can find a nice café at the end of this street.
In the end means finally or lastly. The phrase in the end can come at the beginning or the end of a sentence:
- We looked in many stores to find a nice sofa. In the end, we bought one at the furniture shop at the mall.
- I couldn’t decide what to do last weekend and in the end, I just stayed home.
- Jack was considering which apartment to rend. He decided to move to Brooklyn in the end.