I have a TCL R635 and a Google Pixel 3a. Here's how I got Cast working: Install the Roku remote app onto your Pixel. Start it, detect and connect to your Roku TV, leave run in the background. Start YouTube and now there's a CAST icon. Boom done!
You can do this because YouTube casting is built in. What people are trying to do is mirror whatever is on their phone to Roku. So that if they have a different app that they are playing a video or pictures on they want to be able to see it on their TV screen through Roku. This is no longer supported and does not work with a Google Pixel phone Big jeers to Google.