You can download the installed Process Application Snapshot from Process Server using the following REST API
/bpm/repo/projects/{Process App ID}/export?snapshot={Snapshot ID}
e.g.
/bpm/repo/projects/2066.baab7ebf-a6d4-4f5d-86e5-4f9041a35529/export?snapshot=2064.a9806c98-0ff1-405f-a3ef-e74c8bab662e
the file you will get is named export without any extension you can simply rename it and add the twx extention to it