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I'm not aware of any tutorials as such. I mostly just look at api.jquery.com/ and regularly visit the stackoverflow.com/questions to fix any problems I encounter.

When you say make a custom interactivity available amongst the other interactivities, do you mean as a page type that can be selected? You can't add something as a page type directly, but you could set up a template page in another resource then just import that page when needed, which sort of achieves the same thing.

I am not a devver, but I remember in a long gone past, MS Office used to have a binder to pick up all kinds of office documents and staple them together. Could this be an option to add multiple (different) activities on 1 page?