I hated Arrival. There must be something wrong with me. Help!

That's conservation of detail for you - they quite simply don't need to show the whole thing because it doesn't matter that much to the story at large. They just give you an idea of how the process will generally work, show it in enough detail so the audience grasps how it works in practice, and then trust the audience to accept that it ultimately works so it can move on to other concerns such as the various human-based conflicts and the introduction of the time elements that make up a bigger conflict. Explaining and depicting the whole process step-by-step would take too long and also wouldn't add much of worth to the film in the process.

Also, I thought it was the other way around and that the aliens showed up because they knew that they would need the humans' help at some point in the future, hence it was in their own interest to teach humans their language and future-reading abilities.

I really just want you all angry and confused the whole time.