IDocs are processed by a function module, which are mapped against in table EDIFCT. When you process an IDoc with BD87 you can have a breakpoint in the function and it will stop.
For outbound processing it depends on which program is sending the IDoc. For transactional data like purchase orders, the IDoc is created via RSNAST00 which in turn calls a function module assigned to the processing code (BD41).

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