I read this and understood that you can only be using either biblatex or natbib. However, I have been able to compile just fine with citations without writing neither \usepackage{natbib} nor \usepackage{biblatex}. To compile, I use the macOS terminal and run:
pdflatex
bibtex
pdflatex
pdflatex.
I am using texlive.
So, which package I am using when I don't specify the package? Or how is it possible that my file is compiling just fine when I am not writing specifically \usepackage ?