Operating System of Servers
Linux or Unix is a common choice for the operating system of a server. There are many distributions to choose from. Oftentimes a headless version of Linux is used. This means a version of the operating system without any graphical user interface (GUI) is used. This saves systems resources and electricity, but also means there is no desktop environment and also no clicking around, only text.
Instead, you open a terminal window on your computer and connect to the server computer via SSH (Secure Shell) by typing in an SSH command and login credentials. The terminal window then shows content from the computer you are connected to via the Internet. So, let's say you are using a Windows computer and connect to a Linux server. The SSH terminal you see on your Windows computer shows the files and folders of the Linux computer and you can type in Linux commands to interact with the Linux computer.
Sometimes instead of Linux a version of Windows Server is installed on a server. For example, Windows Server 2022 (there is a new version roughly every 3-5 years). Windows Server looks pretty similar to normal Windows versions but contains server software. Using Windows Server as the operating system is common when business applications by Microsoft are used by a company. For example, Enterprise-Resource-Planning or Customer-Relationship-Management Software. Installing such software on a server (computer) that the company owns and maintains is called on-premise. Nowadays this is less common because Microsoft is pushing customers to use these applications in their cloud offerings. So, between Microsoft and their customers there are incentives and trade-offs regarding the maintenance of software and hardware, licensing/subscription costs and data privacy at play among other things.
There might be other operating systems for servers that I have never heard of in my ignorance. So, let me know if I am missing any popular alternative.
There might be other operating systems for server that are less common or I am not aware of them (sorry for the ignorance).