I'm the guy who built this. I use the windows key to open up the run-time settings menu where you can do things like mount floppy disk images, turn on or off different emulated sound cards (adlib, DSS, covox, midi), switch keyboard layouts, change CPU speed, hibernate, etc. Or long pressing the windows key inverts the pixels. Usually a white background with black text looks a lot better than the reverse, but some programs default to the reverse, so it's nice to have a quick way to re-reverse it. Similarly, I use the "menu" key to the right of the space bar to do a quick refresh of the screen (which redraws all the black pixels making sure they're totally black, in case occasionally some look a little washed out -- rare problem but nice to have a quick solution), or long press to do a long refresh of the screen (where it actually turns all black and then restores the image, which takes about a full second) which makes all the blacks totally black and all the whites totally white. Neither of these refresh functions need to be used very often, maybe once every two or three hours you'll see a situation where you want to use one.
They keyboard is not from a laptop. It's a stand-alone PS/2 keyboard that I removed from its original enclosure and installed in the 3d printed enclosure for this computer. I get them from https://item.taobao.com/item.htm?id=561534245819&skuId=52612... (the "KJW240/10寸PS/2圆口（线长140cm" option). I cut the cord down to about 35cm, since the original cords are way too long, and crimp on a PH2.0 plug which plugs into the motherboard internally. Don't know whether or not you can buy this keyboard outside China. Maybe it's sold on AliExpress too. I tried out a lot of different keyboards before choosing this one. My criteria was pretty hard to meet: must have windows and menu keys, must have left and right shift keys (some keyboards actually don't have a right shift key!), must have both delete and backspace keys (some keyboards don't have a delete key), must have print screen scroll lock, numlock, number keypad (activated by numlock), pause/break, insert keys. Not many miniature modern keyboards actually have all those. It had to be small enough to fit on this computer but big enough to be able to type on fairly naturally. And it had to have pretty low power consumption. This keyboard takes about 6-7mA. Many keyboards I tried took between 30-40mA, more than the CPU itself. The thing I dislike most about this keyboard is the home key being between the quote and enter keys, and the up arrow key being where the right shift key should be. That's a really unfortunate design problem. But its a lot better than a lot of keyboards, and its something I was able to get pretty used to after a few days of typing. Still, I do plan to build my own keyboard to replace this in the future, one with a processor that sleeps between keystrokes so it can consume less than 1mA on average, and that has a standard key layout.
Since you can also plug in a PS/2 keyboard of your choice on the right side of the device, I had to consider the situation where someone plugs in an older keyboard that doesn't have the windows and menu keys. In that case they can use the F11 key to replace the windows key, and the F12 key to replace the menu key, as these two keys were not present on many keyboards and not used by software from the DOS era either.