re: I cannot seem to find it in the WordPress Plugins Repository
Not all plugins are in the repository, there are many “premium”, better defined as “commercial” plugins not in the wp.org. Connections is free and in the repository only because I can make just enough to live on by selling the add ons.
re: so, it made me wonder whether it imports your actual code? (Is that even legal?)
I would venture a guess, no, it does not actually import my code or any other plugins code. Reading over it… it appears all it does is provide a user interface to add third party (ie. Connections) shortcodes to its interface that simply inserts shortcodes into a page.
Is this legal, sure, I see no reason it would not be. The plugin is visual page builder. There are actually quite a few of them for WP, some are built into themes such as Divi.
Here’s a free alternative for example (no endorsement, just the first results in my Google search)
Personally, I do not care for them because they sort of lock your content to them (kind of necessary, really) and add quite a bump in server resources used (which can be mostly negated with a good caching plugin such as WP Rocket), they can also be quirky causing weird little compatibilty issues, but, I definitely see the appeal of such a tool.
Here’s are a few good links discussing visual page builders. I would consider them a good read before your start using one.
hope that helps!