How do they work?
If your TV has an HMDI slot, it goes in there, Chromecast works the same, the Roku Stick comes with a remote, whereas for Chomecast you control it with your cellphone or tablet.
Basically they are a WIFI device connected to your TV. It accesses the internet via your Wireless Network Server/Router.
It then allows you to connect to the same websites that you can access with your PC, but only to those that provide video services.
I.E. You can’t access FR with a ROKU, but you can access NETFLIX.