@leocg None of that worked for me, which prompted me to create a user account, back up tabs and history, and reinstall the whole browser from scratch. That solved the problem. Thanks for all your suggestions, everyone!

@Barkuti I don't usually use the Android YouTube app. I was just trying to watch a video that would continue playing even if I turned off the screen, which I can't seem to get Opera to do. I prefer using Opera because its ad blocking works great on YouTube. All other times I prefer the m.youtube.com site with Opera. I'll take a peek at the app you mentioned. Thanks.

@sgunhouse I think it already is des[link text](link url)elected. Still going to desktop site. Here's a pic.

https://ibb.co/mFJxLf3

@sgunhouse Yes, that was it! Do you know how to undo this?

Problem: m.youtube.com now redirects to https://www.youtube.com/?app=desktop

I clicked the 3 dots menu icon and selected something I don't remember now, but my intention was to have only the one tab I was on open in the Android app version of YouTube. Now I can't find a way to stop the browser from always opening a non-mobile version of YouTube within Opera. I just want Opera to always open YouTube in the m.youtube.com site like it had done before (not in the YouTube app).

I have tried toggling "Desktop Site" on and off.

I have disabled the app version of YouTube.

I have looked all over YouTube for settings to change this back.

I tried Opera>Settings>Site Settings and found that m.youtube.com was set to never open. I reset that but the problem still persists.

Details
Android 10.0 with June 2020 security patch
Motorola G7 Play
Opera for Android version 59.0.2926.53920
Ad-blocker is enabled
Data savings is not enabled