Hi April,
Agree with Michael, Esri Enterprise Administration Class would be a great starting point. That said, I tried to answer some of your questions below.
Portal and ArcGIS Online are the same in that you can create and share layers, maps and apps. The user interface is also very similar, which is by design.
The central difference is that ArcGIS Online is a SaaS offering hosted in cloud infrastructure that is managed completely by a dedicated Esri operations team. Portal for ArcGIS is installed and deployed on-premise, as part of the ArcGIS Enterprise base deployment. In most cases, your IT or network team would be responsible for a lot of the installations, upgrades, etc. that go along with on-premise software. There are also cloud deployment options for Enterprise, which a lot of folks do these days. We see Portal for ArcGIS and ArcGIS Online used side-by-side frequently as well, with Portal used for internal staff and ArcGIS Online used for public or customer facing maps and apps. Here is a full rundown of the two options: Understand the relationship between ArcGIS Enterprise and ArcGIS Online—Portal for ArcGIS (10.8) | D...
>If I move all of my data and mxds to the new 10.6.1 server and publish new services from that server, will they automatically be published in Portal? If so, will everyone who has access to my ArcGIS Online account still be able to see and use everything?
If you have federated Portal for ArcGIS with ArcGIS Server, then the services will appear in Portal. They will not be exposed to ArcGIS Online. Reference: Federate an ArcGIS Server site with your portal—Portal for ArcGIS (10.8) | Documentation for ArcGIS ...
>And, why do I have 80 named users available in my Portal? Can I move them to my ArcGIS Online? Will my named users have access to both ArcGIS Online and Portal?
You can move the named users from Portal to ArcGIS Online. You can accomplish this by calling Customer Service. They will not have access to both ArcGIS Online and Portal.
I hope this helps,
-Peter
