In Opera GX, you can goto the URL opera://settings/early_bird, enable Early Bird, enable "Guest tabs" (disable all the other options if you just want "Guest Tabs") and restart Opera.
Then, goto google.com. Then, right-click that google.com tab and choose "Reload as guest tab". That will open a new tab where you can sign in to a different google account in that tab and goto youtube.com in that tab for example.
That will allow you to have 2 different accounts. Doing "reload as guest tab" again won't help you get a 3rd though. However, you can open a private window (ctrl + shift + n) and log into a 3rd Google account there. That will get you 3 total.
Besides those things, as said, you can enable "Revived GX Profiles" in Early Bird, restart Opera, goto the URL opera://settings/profiles and create extra standard profiles. You can then run those extra profiles at the same time as your main one. You can launch them via their desktop shortcuts or their taskbar icons (if on Windows 11 23H2 for example), or by clicking the menu at the top left of Opera and going to the "Profiles" submenu, or by clicking the profile you want at the URL opera://settings/profiles.
Those are for Opera GX. For Opera Air, you can create multiple profiles this way.
Opera doesn't have container tabs like Firefox does.
The google.com website allows you to switch between Google accounts you're signed in to, but not sure if the site allows you to load special URLs to have all of them active at once. Probably not though.