There are a lot of videos that are NOT on Amazon Prime or YouTube or any of Roku's supported channels. Is there any way to watch these using Roku streaming by simply providing the URL of the website, or is this product locked in to display only for selected content sources?
To watch other internet content (ie. from a web address/URL), you can try using a casting app on your phone/laptop and cast it through the Roku device to your TV. Some of the casting apps that work with Roku are Web Video Caster, Tubio, AllCast and Castify. (These are available in the Android Play Store and can find their companion receivers in the Roku Channel Store. iOS/Apple have similar apps.
When I want to wirelessly (wifi) cast web content to my TV through the Roku device, I personally use/prefer Web Video Caster. Other times, I just load the web content on a laptop browser and then plug a HDMI cable into the back of my TV (bypassing the Roku device altogether).