At both Linux or Mac machines, if you do not have existing key/pair, run command ssh-keygen -t ed25519.
You'll be asked:
Enter file in which to save the key (/home/testuser/.ssh/id_ed25519) → just press Enter.
Enter passphrase (empty for no passphrase) : If you do not have hard-drive encryption, use secure enough passphrase[1] to protect your key. If your laptop is encrypted, you might decide to do not set ssh key encryption, and just press Enter.
Example:
testuser@hostname:~$ ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/testuser/.ssh/id_ed25519):
Created directory '/home/testuser/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/testuser/.ssh/id_ed25519
Your public key has been saved in /home/testuser/.ssh/id_ed25519.pub
The key fingerprint is:
SHA256:SLKcljiOmsdTubEGeG+6QP+nDs0NoLciBA71guz8BXQ testuser@hostname
The key's randomart image is:
+--[ED25519 256]--+
| .. E |
|.o... |
|+..oo . |
|* .+o* . |
| Oo.*+. S |
|+o*oO o |
|++.O * . |
|oo= O . |
|o.o*.+o |
+----[SHA256]-----+
testuser@hostname:~$