question

You build dashboards for the SCOM console in the SCOM console, in the monitoring view. You build SCOM dashboards in the Web Console, by creating a new MP to store the data, and then build it in the web console, from the monitoring pane (right click new view, something like that).

Notifications.internal is a unsealed MP that contains all of your notification information. You can try to import that, it should work, but validate that the current SCOM 2019 environment has NO channels, subscribers, or notifications, because if you import that MP it's going to perhaps replace the existing one.

NetworkDiscovery.internal, sounds like a default mp that gets installed with the product. It might already be in your SCOM 2019 environment, but an updated version.

Advisor.GroupDiscovery, not sure if that is custom or part of the the advisor MPs that ship with the product.

From SCOM 2019 powershell session do the following to dump your management packs:

get-scommanagementpack | select name, displayname, version

That should dump a list of installed MPs and you can compare that to your 2012 environment (you can run the same command in 2012). You may want to add another pipe

get-scommanagementpack | select name, displayname, version | export-csv -path "C:\temp\ManagementPacks.csv"

or you could just run something like:

get-scommanagementpack | export-csv C:\temp\mpdetails.csv