

It is complicated. There are several options, each with tradeoffs in functionality, compatible software, and performance.
A simple method is to use one system as a desktop, and SSH into the others as “headless”.
Other options include making a K8s or HPC cluster (there are other cluster types).
Spreading a single set of communicating processes requires a low latency interconnect. Something better than Ethernet, like Infiniband. But many programs don’t support that.
A person does not need to point out all problems when pointing out one problem.