In the Slicer extension module market, there are many unused modules.
If I want to learn how to use different modules, is there any related document introduction or video demonstration?
Because sometimes I don’t know how to use a certain module and how to adjust the parameters.
The help document of each module is not very detailed.
The extensions are contributions from the open-source community. Some extensions are well-maintained and documented. However, some even become incompatible after a while because they are abandoned. The same wide range can be found in documentation. The community tries to keep the most important ones alive if the original author cannot maintain them anymore (see a few taken over by @lassoan on his GitHub), but this is 100% done in our free time, without any support or funding.
So there is no general answer to your question. You need to go to the website pointed from each extension and see their documentation. If it is not detailed enough or not up-to-date, feel free to contribute.
In case you cannot find an answer, you can ask those specific questions here.