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.