Frontend or Backend which is more complicated ?

I think that the Backend is complicated, due that is the operative part, there are several mechanism that are needed to data process......

Both and depending how deep of a rabbit hole we are talking about.

Backend, but I know that Frontend has challenges.

Backend for most people i guess

It very much depends on what kind of app you are building. In the majority of cases the Frontend will be quite a lot more complicated than the backend.

For me definitely is the back-end. It is quite un-intuitive for me and it can get very complex to the point of making me bored.