You open the campaign using an archiver, go to the Maps/SingleMissions folder, and extract the folders from there. These are the maps, but the videos (if any) will be lost.

It won't work with the developers' campaigns – they are difficult to find.

You create 2-10 maps and combine them into a campaign. How – see the manual.

At the very least, this doesn't work at all in Haven.
And the manual doesn't help with this.
The maps folder, which is packed in the datapack, does not contain ready-made maps. That's the first thing.
Secondly, as the author of the topic already wrote, the drop-down list where the missions should be added to the campaign is empty, even though the maps are single-player maps, and they are for the version that is installed, and they are located exactly where they should be.

So the question remains open and unresolved.

If anyone can help, please do.