Hi,

The development in Odoo12, 13  and 14 are almost same, only there are few changes, as you are exactly looking for v13 tutorials have a look at this playlist in the YouTube channel: Odoo13 Development Tutorials

If you need it particularly for Odoo 12 and Odoo 14, you can access it from the following links.

Odoo 12:  Odoo12 Development Tutorials

Odoo 14: Odoo 14 Development Tutorials

Once you have good basics and understanding of how the things are working, Odoo source code itself is a big text book to learn and develop.  So initially i would like recommend to start like this.

How to Start

Don't directly  try to jump into the development process. Install the odoo in the local machine, either you can do it manually step by step or using the available scripts, if you are using the Ubuntu. In windows the installation is easy, just download the exe file and run.

Once this is done, create a database and what all features are there, how the system is working, understand the working of the basics modules like sale, purchase, inventory and accounting.

For Installation

Ubuntu:

Manual : How to Install Odoo on Ubuntu

Scrypt:  https://github.com/Yenthe666/InstallScript

Windows:

Download and run the exe file from here: https://www.odoo.com/page/download

Also refer the odoo documentation explainingg the same: https://www.odoo.com/documentation/14.0/setup/install.html#editions

Starting Into Development

Once you get a fair idea about the Odoo and its functionalities, try adding some third party modules from the odoo store to your odoo instance and see, it is just to understand the modular functionality of the odoo and to show how customizable the odoo is.

To start development, we need an IDE, for this i will recommend you to configure Odoo with Pycharm, which is commonly used IDE for odoo.

Setting Up Development Environment

Download the pycharm community version for free and run it and configure it with Odoo, you can see the configuration steps here in this video: How to Configure Odoo With Pycharm

Once you are able to run the odoo, you have to understand the concept about the custom addons path, see: How To Add Custom Modules And Custom Addons Path In Odoo

Once the development environment is set and ready, you can start customization by creating custom module, create a simple module which adds the new database table and menus and views.

See:

1. Creating Your First Module

2. Creating New Database Table

3. Creating New Menu and Actions

4. Setting Access Rights

and more you have like Reports, Different View etc.

References:

1.  https://www.odoo.yenthevg.com/

2. https://www.odoo.com/documentation/14.0/index.html

3. https://www.youtube.com/c/OdooMates/videos

What if you get Stuck ?

1. Use Odoo Forum  and create a post, developers around the world will help you.

Thanks