Here are the documents explaining how to customize an ABP project:
Object Extensions
Customizing Application Modules - Extending Entities
Customizing Application Modules - Guide
MVC - User Interface Customization
Customizing Modules Guide
Module Entity Extension Guide
Angular - Component Replacement
Angular - Modifying the Menu
Angular - Page Toolbar Extensions
Angular - Data Table Column Extensions
Angular - Entity Action Extensions
Angular - Dynamic Form Extensions
Angular - Customize login page
Bundling Minification
Customize Login Page MVC
Customize MVC Login Page example
How to Customize the SignIn Manager for ABP Applications
Customize login page of Blazor
Customize register page MVC
Blazor WASM: Customization & Overriding Components
Blazor Server: Customization & Overriding Components
Angular User Interface Customization Guide