You could also use from starlette.staticfiles import StaticFiles.
FastAPI provides the same starlette.staticfiles as fastapi.staticfiles just as a convenience for you, the developer. But it actually comes directly from Starlette.
"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.
This is different from using an APIRouter as a mounted application is completely independent. The OpenAPI and docs from your main application won't include anything from the mounted application, etc.
You can read more about this in the Advanced User Guide.