If you are starting to learn development in D365 then understanding the difference between the customization and extensions is something to investigate. Extension being the new concept to allow smaller changes and better hotfixing procedure in production. This is a simple example of creating an extension to add a new field to a table.
D365FO 1611 – Update3 (7.0.4307.16141)
In a previous post you can see the process of creating a new model and project.