Ask somebody who uses the app to show you how and why they use it. Listen, learn and ask questions, but do not criticise.
After that get some A3 paper and a handful of coloured pens. Go though the script and draw a diagram showing the data flow from inputs to outputs, the outputs being the resultant internal tables that the dashboard viz's will use.
When I have done this it takes a fair few iterations, as the early diagrams will end up with loads of things scribbled out and be a spiders nest of crossing lines. Later iterations will steadily become neater and more understandable. Don't worry if some of the processing looks silly or even insane, they may well have evolved over time with various quick 'n nasty mods - or it may just be that you do not fully appreciate the rationale yet.
From the knowledge gained doing this the data model should be understandable. Do study it especially to find out which tables have loads of / not many rows, plus how the tables join together and the cardinality of the keys.
Open the dashboard to study all the viz's and their dimensions / expressions.
Go back to the person who first showed you how and why they use it, and go through it again with them. Hopefully this time you will understand many of the things that confused you the first time.
Finally try and fathom out what your newly gained understanding will allow you to do and more importantly what should be done and why.