First install openssh-server on the new machine and create the .ssh directory:
$ sudo apt install openssh-server $ mkdir -m 700 .ssh
On the old machine, edit the .ssh/config file and add an entry for the new machine, which I'll call "new" in this example. You'll need to get the IP address of the new machine on your network by looking at the Wifi settings.
Host new hostname 192.168.1.234
Now on the old machine copy these two files over to the new machine:
$ cd .ssh $ scp id_rsa.pub new:.ssh/ $ scp authorized_keys new:.ssh/
Each of those copy commands will ask you to enter the password, which is annoying but we will soon get to a place where ssh will use public key authentication and you won't need to enter passwords any more.
On the new machine, do this:
$ sudo su - # vi /etc/ssh/sshd_config
Now inside that file set these options:
PermitRootLogin yes PubkeyAuthentication yes PasswordAuthentication no UsePAM yes
Write out the file and then run:
# systemctl restart sshd
Now you'll be able to copy files from old to new without entering a password each time. You'll just enter the password of your private key once.