Custom logic triggers something to happen in the system that doesn't already occur by default. SAP has a variety of released Business Add-Ins (BAdIs) that can be added to different applications using the Custom Logic app.
After you've identified a BAdI to implement, navigate to the Custom Logic app to select the correct BAdI. Because business logic is written in ABAP code, the View Documentation link provides information about what is possible to change/control within the code. After you create and publish the BAdI, you may choose to open the Code Editor to view the ABAP code that has been generated.
View the Business Add-In (BAdI) code
If you want to make further changes to the standard code, you can view the sample code that shows you correct syntax in this specific situation. The Custom Logic app is one of the few areas in the customizing tenant where you will see code, but only if you specifically look for it in the Code Editor. In most cases, there won't be any reason to do this, because the code for the BAdI that you select is already generated when you create and publish it. If you make changes to the code that are not supported, the editor simply won't allow you to save or publish. This is a very controlled method of making small changes that trigger other things to occur in the SAP S/4HANA Cloud system with no developer knowledge. After implementing a BAdI, you should always test it out.
Learn more about the Custom Logic app in the SAP Help Portal.