Hi,
Normally the setup will aotumatically do the schema and AD updates for you, so it would be "case 4"- zero times, but personally I would always run those commands manually when upgrading (The computer where you'll run the command needs to be in the same Active Directory domain and site as the schema master. It'll also need to contact all of the domains in the forest on TCP port 389).
Here is notes from official guidance:
The AD preparation tasks are not required to be run separately to the upgrade of Exchange, unless in circumstances where you need to separate the tasks to different teams with different permissions, or if you have a multi-domain forest and want to control the AD changes.
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