Use this library on your phone, tablet, or computer to find the exact video. Then send that video to the YouTube app on your Roku or another smart TV. Roku does not include a general web browser, so the search happens here and playback happens in YouTube.
Install the YouTube app from the TV's app store if needed, then open it. For normal casting, connect the TV and your phone or tablet to the same Wi-Fi network.
Search this library and select a video card. In the video window, choose Open selected video in YouTube.
On a phone or tablet, allow the link to open in the YouTube app. The exact video you selected will be ready to play.
Tap the Cast icon in YouTube, then choose your Roku or smart TV. The TV starts playback while your phone remains the remote.
In YouTube on the TV, open Settings and choose Link with TV code. On the YouTube phone app, tap Cast, choose Link with TV code, and enter the blue number shown on the TV.
You can also use AirPlay from an iPhone, iPad, or Mac, screen mirroring from a compatible Android or Windows device, or an HDMI cable. If you want to use only the TV remote, search YouTube for Lee C Zhao, open the channel @lee_c_zhao, and choose Videos.
YouTube's official TV connection instructions · Roku browser information