can overlay extensions have the same functions as components and panels? are there any special requirements (size etc)?
As I wrote here 20 minutes ago
All slots have the same functionality (ignoring known bugs/issues)
Overlay you can use the whole stream.
Components are essentially panels but you have more size control
Panels are 318px wide and anywhere between 300 and 500px tall (you can choose) these can be popped out to be whatever size the user makes the window.
Mobile is essentially a panel
Is there a twitch website where I can see everything required for an extension?
What do you mean “required for an extension”?
The documentation for extensions starts here Extensions | Twitch Developers