There are plenty of references for generating and using SSH keys on MacOS. Google for answers and they're everywhere. What is harder to do is use multiple SSH keys, especially for the same host, e.g. GitHub. I had multiple SSH keys, and a ~/.ssh/config file, but certain of the credentials weren't being found.

The nuggets of goodness I found describe how to properly use ~/.ssh/config - (here, here, here, here...). The key bit of information to make everything work is to actually use the resulting Host aliases!

Go figure, huh?

In concrete terms, and skipping all the SSH generation tasks — references abound — the key insight in code is...

Add the host-key mappings to ~/.ssh/config,

Host github-alias-1
  HostName github.com
  ...
  IdentifyFile ~/.ssh/key1

Host github-alias-2
  HostName github.com
  ...
  IdentifyFile ~/.ssh/key2

Then anywhere you would normally reference the url host directly... don't. Use the Host alias from the config file instead,

• NO: git@github.com:username/repository.git
• YES: git@github-alias-x:username/repository.git

It's magical.