@JenFrazer_USFS_NR-GISS-IRIN
There are two Map Viewer apps:
- Map Viewer
- Map Viewer Classic
As an organization administrator, you can configure which app you want to use by default on the Map tab by modifying the organization's settings on the Organization tab. Each member can also configure this through their profile.
Individual web maps do have an internal version number supplied by the application that you use to author your web map. Because ArcGIS apps get updated on different timelines, support for new features may come at different times. For example, if we introduce a new drawing option in ArcGIS Online Map Viewer, there may be a delay before ArcGIS Pro releases an update that supports it. Plus, people choose when they want to upgrade to new desktop versions. Thus, a web map may contain an enhanced feature that a given app may not support yet. Web map versions let the apps gracefully handle these differences.
In your example, ArcGIS Collector, now deprecated, doesn't support new features in web maps authored in Map Viewer. Thus, the Collector option is hidden.
You don't particularly need to worry about the web map version number (you can't really see it), unless you are using older apps that require an older web map version to run--like Collector.
Hope this helps,
Mike