Are you telling me Beowulf clusters are back?
Jokes aside, it depends what you want to do. You can't really build one powerful gaming PC out of multiple, but your can run parallel workloads in a number of different ways. What exactly, comes down to what you're doing. A kubernetes cluster is different from a Blender render farm, for example.
As others mentioned you can just remote into the servers with ssh, vnc, rdp, etc. if you want physical displays on them, you can look for a cheap KVM which lets you control multiple PCs with one keyboard, monitor, and mouse.