As stated earlier, everything except the handcrafted code is generated as it would be if the entity had been defined OOTB.
In the case of a custom extension of an OOTB entity, these handcrafted implementations will already exist. The generator creates handcrafted classes in the custom source package which are modeled using the replace superclass option, where the superclass is the existing OOTB implementation. These will only contain method stubs each of which will, by default, begin by calling the superclass implementation.
In the current example, the handcrafted preCreate function will need to be updated by the developer to assign the value of the username attribute to the creation struct. Also, the handcrafted validateDetails function may need to be updated to ensure the 'reference text' field is not left blank.