I know iPads (and I assume Android tablets) can be a second screen over wireless using third party software but it’s not uncompressed video with disk access last I checked.
The video is compressed (how much depends on your network speed, it's not always noticable). And it's far more than just video - you can copy files over the connection. Keyboard/mouse/touchscreen/stylus inputs are sent over it, and video camera/microphone data can be streamed in real time as well. There's also a control protocol to temporarily switch from sending the entire screen to sending just a URL (and auth cookies) to a HLS video stream such as a YouTube video - which will cause the other computer to directly access the content over the internet instead of one computer downloading it, decompressing it, then recompressing it and sending it to the other computer.
And it's not just iPads. Macs, iPhones, Apple TV... they all have that capability. It's the core underlying system behind AirPlay, AirDrop, Continuity Display, Universal Control, Clipboard Sharing, Continuity Camera, etc etc.
I do it all day every day between my desktop and laptop Mac — I effectively use this as a KVM so I can control my laptop using the nice mechanical keyboard and mouse attached to my desktop (also, it's a handy way to avoid having to keep data in sync over the cloud... I tend to do all my note taking on the laptop and just never access them from the desktop - eliminating any risk that one of them might not be fully synced up with the latest data).
It works best over thunderbolt but it's usually done with wifi — always a direct wifi connection that bypasses your router because the amount of bandwidth required is so high that if you sent it to a router and then to a computer... your wifi would almost certainly collapse under the load.
Target Disk Mode doesn't exist on modern Macs. It has been replaced with a new "Mac Sharing Mode" which is technically completely different. The new system is basically just a regular network fileshare (I think it uses SMB), while I think the the old system was PCIe connection if you had thunderbolt/firewire (fast) or something much worse if you were using USB (that never worked well).