i totally agree with you, it is complicated, too many things to consider, it is not easy to know where to start.
There should be guides like; if you want to do this start from here and follow these steps;
for example if you want to develop your own module in erpnext, start from here and follow these steps,
if you want to add custom fields start from here and follow these steps, etc etc.
Development mode, server side script, client side script etc. they are complicated.
i want to do this;
Product Specs section - ERPNext / Customize ERPNext - Frappe Forum
but i dont know where to start, it should not be that hard.