Her is my take to try to help you.

If it’s your own code, you can add docstrings comments to your functions, so you don’t have to re-read the function body everytime. Also, name functions to be understandable more easily when possible.

If not your code, write on a piece of paper (not on computer) the in and out of a function, maybe like so:

 [1,2,3] -> (sum function) -> 6

Then, you can even connect the functions together and see the whole algorithm:

[1,2,3] -> (sum) -> (multiplyBy2) -> (...) -> final_result

When projects get more complex, paper will not cut it, then some not taking app of some sort will help. (logseq could help, but some mind mapping or sequence diagram programs would help as well)

Also, I don’t know what language your are working with, but learning LISP (maybe clojure) could help.
Why? Because you have to connect your functions together, and it forces you to do so.
At first, it might be harder compared to what you’re used to, but it’ll give you better fundations to keep learing.