For a long time, I’ve just put on DejaVu fonts and been done with it. Generally good enough Unicode coverage for me. But I know it’s been years since DejaVu’s been updated, and I wonder what’s very common today.
[As for the terminal, I’m guessing it’s usually still the standard fixed Unicode fonts?]
Ubuntu uses their own font family. I think it’s one of the only distributions with its own custom font, but I might be wrong. The Unicode coverage of the Ubuntu font is not very big compared to Google’s Noto font family, which many distributions switched to as default. But it mostly depends on the DE — Gnome uses the Cantarell font, KDE uses the aforementioned Noto font.
Well it’s pretty much the entire point of Noto after all, so it’s probably hard to beat, from the website: