Mac storage basics · Apps and macOS itself
What are login items and background items, and do they use storage?
Short answer: Apps that open at login and helpers that run without a window. Manage them in System Settings → General → Login Items; they use little storage but leftovers can keep running.
Login items are apps that open when you log in; background items are helpers apps install to run without a window (for updates, syncing and similar). You manage both in System Settings → General → Login Items & Extensions (named Login Items in some versions).
They use little storage themselves, but a helper left behind by an uninstalled app can keep running or keep writing caches. Turning off ones you don't recognise or need is harmless; the app may ask again next time you open it.