How to import hierarchical objects of project billing contracts with REST API ?

Summary:

We want to use the REST API to import project billing contracts (https://docs.oracle.com/en/cloud/saas/sales/faaps/api-import-activities.html). It is not indicated in the documentation how to import all the file hierarchy (header, lines, contacts, associated projects, etc.) in one call the same way we can do it in the UI.

Content (required):

We have multiple files to import so we can create a project billing contract fully operational. With the official documentation, it seems to be possible to import files one by one but it sounds ineffective even though it is possible using Import Management UI.