Pixels are tiny squares dots that have color filters to display what is on the screen.
These squares are used to draw smooth curves for everything from letters to images. Obviously squares are not very smooth.
Displays with very high pixel density would have squares so small you can't distinguish them so objects would appear more smooth but of course smaller.
Displays with lower pixel density would have more jagged edges because larger squares are used to make curves but the image would appear bigger.
Smooth edges but smaller picture, rough edges but bigger picture. Im not sure what you consider better "clarity" but I personally prefer my edges smooth.
Theres plenty of screen with glossy coating. They are almost everywhere. Usually on TN panels though because they have bad color. Be aware of the highly reflective nature of glossy displays. If you have a room with a window, you better have some nice curtains otherwise the monitor would be useless during daylight. You may not notice with a cellphone or laptop where you can hide from the sun but there is no escape for a desktop monitor.