How do I write a plugin?

I found the SDK page: https://www.scirra.com/manual/15/sdk, which has a template but no information on how to write plugins.

I could not find any tutorials, nor info in the main manual.

E.g.

1. how does the plugin include external js libraries for runtime only (I only see common.js as the way to include libraries)

2. how to pass multiple parameters in/out?

3. what is the difference between a c2addon and a plugin?

4. How the plugin and C2 runtime and C2 editor interact.

The particular plugin is runtime only, no layout stuff.

Any suggestions as to resources or tutorials? Perhaps some hello world and slightly more complex examples?

Update- by luck I found the SDK manual, completely hidden! Its actually on this page: https://www.scirra.com/manual/15/sdk but what I didnt notice was there on the left, you can click on the word "SDK Documentaion" and there is the info in the dropdown. They should have put a link to it in the sdk page, like "start here" or "goto manual"