Any packages you want to use will need to exist on paths on the various ROS environment variables (e.g. PATH, AMENT_PREFIX_PATH, etc). The order in which you source these should be determined by what you want to have priority.
source /opt/ros/galactic/setup.bash

You can add these two lines to the end of your .bashrc if you don't want to manually source both paths every time you open a new terminal.