Where does the sudo .bashrc file reside?
I prefer to use sudo --set-home and/or sudo --login when I care about the dot files so that the environment gets set up more like the normal user than the restricted environment.

This seems a bit like an XY problem though, it really depends on what you are trying to do if setting up a proper root user with a proper environment is needed or if you can modify the sudo behavior using standard flags and functionality.

You can find the home folders generally with this set of commands:

sudo -s
who am i
whoami
pwd


In the first case, you are the normal user with root - sticking to the old user home folder and dot files. If you instead do sudo --login you'll see the home folder for root. By default it is /var/root