City plans are a core feature of Sim Settlements 2's mechanics. They are settlement designs created by the community that settlers can build themselves so that players don't need to build structures piece by piece (but they still can if they want to).

Check the Sim Settlements 2 Wiki for details for details on city plan mechanics.

Unknown settlements with a formkey in the label, such as "Unknown: 000001:MyPlugin.esp", are usually mod added settlements. I try to catch these and add the proper name of the settlement so that they display correctly. If you see one, feel free to report it to me.

Unknown settlements without a formkey, such as "Unknown:", are the result of an issue I'm currently have when collecting the data from the plugin files. There is no need to report these. I will have them fixed eventually.

Currently, screenshots aren't likely to be added for a couple of reasons.

There are over 700 city plans at the time of writing this. Screenshots for the other items cataloged on this website were possible because I was able to create scripts that automate the process for me. This not so easy to do with the city plans.

Many screenshots require other mods outside of the SS2 ecosystem. This creates mod conflicts and prevents me from being able to load the game with them all enabled, even if I could automate the process.
Modded settlements are probably the worst example of this as the settlement mods often require different pre-combine handling mods.

At best I might take screenshots of vanilla settlements that only require SS2, but it is not a high priority.