I am creating custom fields that will be part of customers data. The fields are:
Car Make (Toyota, Honda, Ford, etc)
Car Model (Corolla, Civic, Ranger, etc)
Car Regisration Number
For field number 1 and 2, they are related. The car model will show only if its related to the Car Make. For example, it will show only list of Honda model, if we choose Car Make → Honda.
How can I do this in ERPN?
You need to write client side scripts for dynamic options.
For starters follow this link: https://frappe.github.io/frappe/user/en/tutorial/form-client-scripting
In the page, from the link you gave, it mentioned:
In the Library Transaction DocType, we have only field for Member Name.
I open DocType page, there is no such thing as Library Transaction on the DocType list.
It is a tutorial for making a Library Management System with Frappé framework. It is not a part of ERPNext.
You can follow the tutorial to learn how to attach form events.
Do you have the tutorial link?