Michal
this is indeed a tricky one! I will try to clearify your questions. 1st of all: you are missing nothing so far. Alle your links are correct. especially the dashboards available can be found in here and the instructions how to create a new one based on your own AQL query for instance. There is no repo available on App exchange specializing on dashboard json files. The screenshot shows what has been provided so far by Jose. Thanks again to his fantastic contributions. Each content extension comes with new features that can be used in your Qradar rules, dashboards, reports and searches. An extension contains multiple object types. In many cases the existing features will just get expanded, like in your sample regarding linux. So your existing dashboards may contain new features as in your linux example. The base function is import and export your custom dashboards as json files. Please search for "dashboard" in app exchange without focussing on content extension. You will find new functions regarding dashboards like here https://exchange.xforce.ibmcloud.com/hub/extension/f4a537a424977e155105d8aa9f5283c3
Depending on the type of extension you install, there is no general approach, it always depends. Pls use the documentation provided. The "fantastic capabilties" are kind of marketing language - sorry for that. Everything said in here of course is correct. The linux custom properties and the GDPR dashboard are very much independent from each other. If you got more specific questions pls dont hesitate to ask!
BR Karl