How do people build Internal Executor GUI?

I don't see this kind of releated stuff much on the internet but, am curious on how people actually build something that on injection, allows the user to have a internal executor GUI customizsed by the creators. I'm looking forward to build/watch how people do it!

Internal Executor GUI example from Madium Executor:

https://gyazo.com/ac41b3c7efcc5eaae2a598f921a06fca