There is a ton of ArcGIS Online reference material available. This document outlines the various content platforms and how to navigate them to discover answers. These platforms are covered:
Please let me know any feedback in the comments, and I hope you found this helpful.
ArcGIS Online Help Documentation
The ArcGIS Online Help Documentation is the first place to look when searching for an answer. It provides a breakdown of each component of the ArcGIS Online platform. Select a category and expand the left-hand menu to find the topic of interest.
ArcGIS Blog
ArcGIS Blog posts provide in-depth workflow descriptions, examples, and announcements about new features. For ArcGIS Online, the product team writes the majority of the posts, so it's a great way to learn about features right from the source. To find ArcGIS Blog posts on specific ArcGIS Online subject matter, the optimal way is to browse to ArcGIS Blog and select the search icon to expose a filter dialog. Within the filter dialog, select ArcGIS Online in the Product drop-down and the topic of interest in the Category drop-down. If the subject is not listed in the Category drop-down, leave ArcGIS Online in the Product drop-down and ensure the Category drop-down is blank. Then, search all ArcGIS Online blog posts by keyword.
GeoNet Blog Posts, Documents, and Answered Questions
There are a lot of blog posts, documents, discussions, and answered questions on GeoNet about specific workflows, data, issues, and customization. If the question "I wonder if someone has done this before" arises, GeoNet is the place to go. To reach the ArcGIS Online content, browse to GeoNet home, then select Communities > Products > ArcGIS Online > Content. From here, use keywords and tags to search for content. Optionally, filter the search by content type from this dialog as well.
Support Knowledge Base (KB) Articles
Esri Support has thousands of helpful articles in their knowledge base. These range from FAQ about platform updates like TLS 1.2 in 2019 and the upcoming HTTPS enforcement coming in December 2020, How To articles that serve as step-by-step guides for workflows, and troubleshooting-centered articles that detail how to resolve specific problems. The most efficient way to search the support knowledge base is by keyword. Browse to the knowledge base and type in the subject of interest, then optionally filter by date published and sort by article age.
ArcGIS YouTube Channel
Esri Product Teams, Events, Support, Distributors and other teams publish a lot of technical content to YouTube. To find ArcGIS Online videos on YouTube, search for and select the ArcGIS channel, then choose Playlists, and scroll to find the ArcGIS Online playlist. There are other playlists to check out in the ArcGIS channel as well.
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We hope this document helps to discover more ArcGIS Online answers. Again, please let me know any feedback or questions in the comments below.