I am doing system and server administration for 3 years and have experience in windows and linux systems.
Stuff I worked on:
Windows Server 2012
Active Directory / Domain Controller, DNS, Microsoft Exchange Server
Linux Server deployment and administation
Mostly RedHat / CentOS 7/8, but worked on other distros as well.
Network administration (Cisco IOS)
Virtualization (VMWare, VirtualBox, libvirt/KVM, Hyper-V)
Mail Servers (Postfix, Dovecot)
DNS Servers (Bind)
Chat Servers (Openfire, Gajim)
Identity Management Servers (FreeIPA / IDM)
VoIP Servers (Asterisk, FreePBX, Elastix, IssabelPBX)
NTP Servers (ntpd, chronyd)
As well as all the common admin jobs like client/user administration and physical work with computer and network equipement.