Another example is the use of commonly known constants, such as pi, i or e. No paper will define those constants and explain indepth what they mean, so you need prior knowledge or to find external resources. I think for such complex papers you will find many such cases where extensive prior knowledge is needed.
I sometimes rewrite them using A, B, C then try to understand them. This procedure should be automatic but unfortunately TeX is ancient.
Can't be harder than learning the meaning behind these characters: https://www.pandatree.com/book/DiaryofWorm.jpg
It's like watching a Marvel movie and not only knowing the plot of the current movie but also the deep history of each character and their relationships with other characters.
I assume the paper didn't come out of nowhere and it's based on "the shoulders of giants".