In activiti 5 the forms are generated throug the deployment of the bpmn.xml all the settings of the form are specified in the eclipse modeling tool, beside this you can make your own forms with any technology you want, the form will be developed to your needs, manually.
Im now inspecting the version 6 of activiti An its amazing, you could create a proces and after that create in a visual way, like drag and drop, a web form an bind it to some task of the process, its very nice.
I hope my answer helps.
Best regards from Mexico.
