Populating content based on fixed DB?

How can I pre-populate the contents of a DB based on pre-existing content (rather than the user having to create it)

E.g: “projects” contain “kit parts” so if someone creates a project, I want to be able to pre-populated recommendations for them based on a DB of kit parts that are assigned to that category of project.

I can’t picture the entire thing you’re trying to do but this is possible.
You need a DB/Collection of subcategory and DB/Collection of main Category.

DB Main Category many to many relationship to Sub Category.