Create Custom Excel Formulas ASAP

I have an existing Google sheets workbook that was built by someone else with far more excel experience than me.. The built-in functions alone won’t get me the results I’m after, so i need to expand on it and develop it further.

Everything is already laid out in individual sheets and the formulas are already in place. I just need them rearranged..

Timing is critical: I’m ready to hand over the workbook immediately and would like the finalized file back, fully tested, as soon as possible. A quick walkthrough of the changes (short Loom video or concise notes) will complete the job and let me hit the ground running.