Export synthesized top-module and import in another project

Try this wording: the developer project is where you create the top-level design that turns into the root_partition. Think of it like swiss cheese: there's a big hole in the middle that will be filled in with the internal function of the design. You then have a consumer project that takes that root_partition where the hole is filled in.

Your project letter names you mention don't designate which project will be the developer project where the root_partition will be designed with multiple holes for the other parts of the design. Once that is done a consumer project gets created from it and the holes are filled in with .qdb files from each person working on a different part of the design. Each of these people will have to honor the Logic Lock region assignments defined in the developer project so that their part of the design is placed in the correct hole in the swiss cheese.

You have to create the top-level design and root_partition.qdb in its own project to make all this happen.