Thanks for the explanation and I still have a question regarding this. Is the hidden part of the equation selected because it has small impact for the whole equation, or the software (Wolfram Mathematica) simply hides random part of the equation because it is considered would take too much space?

Because in my case the hidden material is kind of unclickable thing or let's say I couldn't find the display more button anywhere.

Look at the help of Skeleton. What version of Mathematica are you using? The behaviour of seeing skeleton objects changed a little bit over the years. With the newer version you never see them like you do in your screenshot unless you use the function Shallow or Short explicitly.