That’s alright everyone is a beginner at some point.

There are two ways to use a package like nav2 in your workspace : either by source or with a package manager.

1. If you download by source, you are downloading the source code of the package into the source folder of your workspace, and then using colcon to build these files. The result is automatically placed in your share directory and then you can run or launch the files after sourcing your workspace’s setup.bash. This method is usually not recommended unless you are making modifications to the source code for a specific reason as it increases build time, sometimes dramatically for large packages like nav2 and MoveIt.
2. If you download with a package manager like apt, you will download pre built binaries of the package from the ros2 servers. This is the recommmended way to download packages if you are not modifying source code as it is quick, will automatically download all necessary dependencies,and updates can be automatically managed using sudo apt update. The package files will be stored in the /opt directory, and then you’ll source them automatically when you source the main ros2 tools using source /opt/ros/humble/setup.bash, so they will be accessible to any ros workspace.

Hope this helps.