As well as considering resolution the next key measure is response time - especially for gaming oriented pc owners.
Then there is refresh rate. Most common is 60 Hz. But there are also 120 and 144Hz monitors available. Again usually the gaming enthusiasts look fo these to be able to use high fps they ca get with top end gaming pc's.
Next consideration is whether to get IPS or TN panels. The IPS are favored for coloring and clarity by especially graphic design and video makers. But they usually have poorer response times.
1)
You can produce any colour with a combination of red, blue and green light.
Most LCDs work by backlighting with white light and colouring with red green and blue for each pixel.
2)
This is called a tiled display. The first 4K screens were actually two 1920x2160 screens put together.