Files can be added to ArcGIS Online via the Content menu, Map Viewer, and Scene Viewer. We recommend adding files via the Content menu since it supports more file types and customization options than direct uploads into the Map or Scene Viewer. Files that are uploaded into the Content Menu (aka My Content) will be accessible afterward as layers in both the Map and Scene Viewer.
Check out the following resources to learn how to use various analysis tools in ArcGIS Online. The Join Data page of this guide provides information about how to perform attribute and spatial joins.
The following tutorials provide a helpful introduction to using ArcGIS Online.
ArcGIS Arcade is an expression-based scripting language that's integrated into ArcGIS software. You can use Arcade expressions directly within ArcGIS software to perform calculations, clean your data, customize the formatting of your pop-up windows / text / labels, and more.