Problem with installing Widevine

I'm not sure If I'm sure I get the ways to get widevine.

Could please someone create an excerp and tell me the prefered way to install it?

As a general rule you don't need to directly install Widevine. If you install add-ons that require it they will use inputstream.adaptive to handle the playback of media streams and this add-on will initiate the installation of Widevine libs. All you see is an on-screen prompt to agree to the download of them.