learning by doing
Theory and practice are both important. It is crucial to understand how IP networks work, so I'd start with book like "TCP/IP Network Administration"
During installation
server software was installed on *ONE* of them. Communication will be
via WiFi.
Did they get IP addresses from the DHCP running on your wifi router or did you set IP addresses manually?
Use the "ip" command on each laptop to see which IP addresses do they have.
If addresses are ok, then use "ping" to see if laptops are able to "see" each other.
Then run server software on one of them (apache web server or nginx for example) and try to open it from another laptop using browser and http://[laptop_ip] URL.
After that, you can play with other server software (like OpenSSH which can also be installed with tasksel) and "ssh" from one PC to another.
Then you can try to configure firewall (using iptables or nft): "linux network administrator guide" can be used to study iptables.
Or study how DNS works (See "DNS and BIND" book)
As MTA (electronic mail) I prefer Postfix (which is available in Debian) and the book named "The Book of Postfix".
These are "core" services each network administrator must know.
But there are alot of other services like samba, ftp, databases, etc