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.

parallel computing cluster of networked commodity computers

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connect them all to one display

The X Window System was invented originally with this idea. It can even connect multiple machines to multiple displays seamlessly.

en.m.wikipedia.org/wiki/X_Wind…

It still works fine, but after most people have been using it only on 1 PC and screen, they abandoned it in favor of Wayland.

Nowadays even self hosters, the cream of the crop of all nerds usually prefer headless servers and only 1 PC with many screens connected to it.

You don't need to connect them to a display. Give them power and network, and access them remotely via ssh (or graphical protocol if you really want to, but unless you specifically want to run interactive GUIs there's not much point).

As for combining their power... it depends on what kind of work you want to do.