As you already know, with Microsoft Dynamics 365 Business Central SaaS, you don't have access to the database or to the standard base code (this is different in the on-premise version, where you can still have access to the base code, and modifying that core is your responsibility). In the SaaS world, you cannot alter the database schema and cannot alter the standard business logic.
In the previous versions of the Microsoft Dynamics ERP, we have always talked about code modification. In the SaaS world, we have to start thinking about a new concept: code extension. To customize Dynamics 365 Business Central, you have to create extensions.
An extension (according to Microsoft's guidelines) is defined as an installable feature built in a way that it does not directly alter source resources and that is distributed as a preconfigured package...