How to Create Field Trigger for Purchasing in Application Composer

Summary:

I want to create trigger(s) for Purchasing through the Application Composer after I've done some reading through the internet and watch some videos about the Field and Object Trigger. Most of the video I watch only created for Sales, and I want to create for the Purchasing, and I cannot find the Purchasing object in the Application Composer. Do I just create a custom object for Purchasing?

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