To start off you generally start from scratch, maybe import a model from the base game for comparison to get the scaling correct.
Afterwards I tend to keep like buildings in the same blender file so I can borrow elements off of it, it aids in set cohesion when you can compare things side by side
All the my flora is in one file as well, but the only things to borrow between those are leaf planes, tree trunks to edit slightly.
its better to start from scratch so you have a deep familiarity with the meshes, as a project space can get quite hectic even with that in mind
cheers, and good luck