ArcGIS Online: is a fantastic online mapping platform for most users, but it's just a part of an ecosystem of GIS Software. Functionality increases quarterly, but it has limited functionality compare with ArcGIS Pro, ArcMap and QGIS.
ArcGIS Pro and ArcMap: desktop applications installed on a Windows machine. Both these offer greater functionality than ArcGIS Online. If you on a Mac (iOS), use the CloudLabs listed below. Download these using your @ucsd.edu Google account from lib.ucsd.edu/gissoftware
QGIS: an open source GIS desktop applicatoin that is very similar to ArcGIS Pro and ArcMap. This works on Windows and iOS environments.
cloudlabs.ucsd.edu: virtual machines provided by campus ETS that provides access to ArcGIS Pro and ArcMap
Create ArcGIS Account