I am not an expert

If you were, you would know that a website has no way to see what extensions you have installed, unless those extensions modify the page in a way that JavaScript can detect.

Radar has an API, and there are mobile clients that allow you to add movies for download. I use LunaSea, it works with Radar and Sonarr, among others. Then I use Jellyfin to stream it to my phone, wherever I might happen to be.

Either Bottles or Lutris. Lutris is the popular way, but the Bottles fans swear by it. Either one will get you going, and neither one is usually difficult. There is the occasional game or app that is a little bitch though.