Some examples from my .ssh/config file.
All my automatically installed Raspberry PIs are in the same tinc within the same group of ten. I use archlinuxarm, so I want to use the user alarm as my default. The IP address range is from .20 to .29.
Host 10.10.10.2? User alarm
On the rare occasion I have to manually create an AWS server, I need this.
Host manual-aws-server HostName ec2-xx-xx-xxx-xxx.eu-west-1.compute.amazonaws.com User ubuntu IdentityFile ~/.ssh/given-certicate.pem
Login with custom user, custom port and specific certificate only for this system.
Host custom HostName custom.local User username Port 8022 IdentityFile ~/.ssh/custom-local-id_ed25519