That's a wide field, surely not a Gordian knot to solve. I don't do anything but occasional cleanups.
mostly too much camera/gallery garbage: --> settings, apps, all, App info, clear data/cache
sometimes too many leftovers from uninstalled apps: requires CAUTION and a root file explorer
could be a corrupted SDcard, which needs a reformatting/replacement
Anyone got Pokemon Go working?
Unable to authenticate message error.