When bash initializes a non-login interactive bash shell on a Debian/Ubuntu-like system, the shell first reads /etc/bash.bashrc and then reads ~/.bashrc.
The reason that /etc/bash.bashrc does not appear in normal bash documentation (such as here or here) is that it is a feature added by Debian and adopted by Ubuntu. As Debian explains it (readme.debian):
What is /etc/bash.bashrc? It doesn't seem to be documented.
The Debian version of bash is compiled with a special option
(-DSYS_BASHRC) that makes bash read /etc/bash.bashrc before ~/.bashrc
for interactive non-login shells. So, on Debian systems,
/etc/bash.bashrc is to ~/.bashrc as /etc/profile is to
~/.bash_profile.
Thus, /etc/bash.bashrc is documented in the Debian and Ubuntu man pages but not in man pages from other distributions that do not support this feature.
/etc/bashrcanywhere. – mikewhatever 13 hours ago