These plugins are example plugins built by the RemNote team to demonstrate how to build plugins using the new plugin SDK. The source code for each plugins has been documented to explain the various API functions.
If you are interested in building your own plugins, taking a look through the source code for these plugins (as well as the plugins built by the community) would be a great starting point. Of course, you should also check out the official documentation, guides and tutorials on our plugin website.
If you are new to writing plugins, we recommend checking out the dictionary plugin project tutorial.