Hi,

   You can add the functionality to existing model.just refer this link for further developmentDeveloper Docs           

Hi,

As you are looking to add some new functionalities to the Odoo, you have to create a custom module for that purpose. There is some people who get into Odoo code and make changes inside it. This practice is not a recommended one.  The better way is creating a new module by your own and add the functionalities to it.

For your case,  you can either create a automated action or a cron job to send the customers remainder email based on condition. For creating this either you can do it from the front end or by the code. From the code it will be better, as you can use it across different database.

For sending the email using the automated action see this:-

1.  Automatically send emails

2.  Odoo Automated Action and Server Action with Email Send

As you are starting with the Odoo development see this, might be helpful in the coming days.

1.  How to Create a Module in Odoo

2. For making changes in existing views: https://www.cybrosys.com/blog/adding-custom-fields-to-existing-views-in-odoo-v12

3. How To Create Custom Module in Odoo12 Easily Using Scaffold

Thanks