Use the Include directive to merge an external config file in the main (default) ssh config file.

1. Create another config file as usual.

2. Add the following line at the beginning of your ~/.ssh/config file
   

    Include path-to-new-config-file
   

   for example, if we have a config file in ~/Documents/.ssh/config add Include ~/Documents/.ssh/config in your main ~/.ssh/config file

What is this good for?

As I mentioned in How to use multiple SSH keys on a Mac with Github or Gitlab my objective is to s separate personal projects from work projects. In this case, I can use an ssh config file for work and another file for my projects, and then include my projects into the main config file.
This also allows me to track my changes with a private git repository without worrying about confidential information from work.

References and further reading

•  SSH Config Includes
•  Can you have more than one /.ssh/config file?