Do it in every terminal where you will start simulation:
(for PC IN LABORATORY)
export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp # for every terminal
(for PC IN LABORATORY)
export ROS_DOMAIN_ID=1 # please use PC number
(always)
export TURTLEBOT3_MODEL=waffle
(always)
export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:/opt/ros/foxy/share/turtlebot3_gazebo/models

Download the YAML config file to modify & add plugins (for PC IN LABORATORY):
wget https://raw.githubusercontent.com/ros-planning/navigation2/foxy-devel/nav2_bringup/bringup/params/nav2_params.yaml

If you are working on your own PC - you can find this YAML file there:
/opt/ros/<your-ros-distro>/share/nav2_bringup/params

Command to start simulation with TB3 and Nav2 stack (warning: it will take a while to start, up to 30 seconds! remember about setting initial pose with 2D Pose Estimate):
ros2 launch nav2_bringup tb3_simulation_launch.py headless:=True params_file:=$(pwd)/nav2_params.yaml

List of Nav2 plugins in Foxy (added during the move from ROS 2 Eloquent to ROS 2 Foxy):
https://navigation.ros.org/migration/Eloquent.html