I like to use my 3 laptops as secondaries screens only, as multiple monitors, to use laptops as screens, and drag windows running on my desktop to the laptops. What the easiest way to do that?
You may try xdmx. Basically it will allow you to use an old laptop as an extra monitor. Here are some links: http://dmx.sourceforge.net/ and real life application https://www.youtube.com/watch?v=YOzRCBGDVaE
Second option for You is Synergy https://symless.com/synergy for sure it shares one mouse and keyboard between multiple computers
What you're trying to do, with the little information you have provided, it's going to be difficult. With the description you have given it's likely that you expect the regular user experience and dragging a live application from one running system to another isn't easy.
A laptop can't be attached directly as a secondary display to your system. Each of those devices is independent and will need to run its own OS and probably some software to get you part of the experience you want.
On option would be synergy which just would allow you to freely move mouse and keyboard from one system to the other without requiring a hardware switch. As you're running Linux it might be possible to find some solution that's going to allow you to view a virtual screen or maybe even just the Window of an application on another machine.
But it's unlikely that it will allow you to just drag applications from one machine to another. You will likely have some form of user experience break.