I have this one line as the opera repo
deb https://deb.opera.com/opera/ stable non-free
And yes, it is installed with opera's deb, if you answer yes at the question "do you want opera to update with the rest of the system?".
As for extensions, I use
ublock origin, because the built-in adblock fails to catch a few ads
download chrome extension, so as to be able to use chrome's extensions as well
autoscroll, because all chromium based browsers on linux lack autoscroll