You can open files, but you cannot create new database files and you cannot make design changes to a database file.
Here is what I want to do. The user may have, lets call them, Projects. Each is a stand-alone database of information. Basically, I want separate file sets (folders) for each project as they often have nothing to do with each other, can be quite large, contain hundreds (or thousands) of externally stored container files, and need to be shared or copied to others on the project.
Whether using a runtime version of FileMaker or the regular, this does not sound like the optimum design to choose for your database. Why do you need separate files for each project. Data for multiple projects can be easily stored and managed in the same set of tables in a single file.
But I think that you can use Save A Copy As to save a copy of a database file with the Runtime. By saving a copy of the file as a clone, the copy could import a selected subset of records from another file such as the original copy--but this is not a design strategy that I can recommend from what you have posted thus far.