A default user name and password has not been used for several years. Quite some time back California added laws regarding default passwords, and before that there was either default name/pass of “ubuntu”/“ubuntu” or “nvidia”/“nvidia”, but this no longer exists. It is now entirely in first boot setup.
Note that prior to flash, if you choose, then on the host PC’s “Linux_for_Tegra/” location, you could add this content to avoid needing first boot setup (just don’t do this if shipping commercial products to California):
sudo ./tools/l4t_create_default_user.sh