Barrier

If you ever need to control another computer or other computers from your own, you should consider Barrier for this task. You can control multiple devices, even if they are running Windows, macOS or Linux.
After you have connected the machines, you can start using the other(s) with your mouse and keyboard. Just like having multiple displays, you can move your mouse cursor from one desktop to the other and then your peripherals will start working on the other device.

How to do it

You might think it can be complicated to do it, but it is not. You just need to install the program on all the machines you wish to use and connect them via Barrier.
When you're done, you can select which side of your screen to move the mouse cursor on in order to enter the other desktop(s), and that is it. Whatever button you will click or press from then on will take effect on that computer, as long as you keep the cursor there.
The program also allows you to configure several useful settings. For example, you can establish hotkeys that you can use on your main device, or on any of the others. Another helpful tweak is to set up dead corners, to make sure your cursor can't get through them to reach other desktops.

No internet connection needed

If connecting all the devices to the internet would not be possible, then you do not need to worry. All you need to establish a simple local connection between your computers.
And regardless if you do this with or without the internet, the connection between the machines is very smooth and it doesn't lower the performance at all.

A negative point

If you want to copy content from one computer to another, that may only work for text. The application should support the drag and drop file transfer, but we could not make it work while testing.
Even though the program isn't perfect, the fact that the connection is almost unnoticeable, along with the other aspects, makes it very good to have.