I don't believe there's any such channel. Some Roku remotes offer voice control, but it has nothing to do with Google/Alexa. If your Roku device doesn't have a voice remote, there's no way to add voice control. If your Roku did not come with a voice remote, some models support adding one. But some devices are IR control only, so the Voice Remote isn't an option at all.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.