Why does my sourdough starter have in the top the liquid separated from the flour?
That is referred to as "hooch". It is water and alcohol. It forms when the yeast has passed its peak activity. I've noticed that it corresponds with the increase of bacterial activity when the starter gets more sour.

The hooch will not form when you are feeding regularly. It is harmless and can be discarded or mixed back in.

Sourdough is a balancing act. Finding the feeding rate that will produce the lift and flavor you want. Slower feeding will make it more acidic and rise less. More frequent feeding will do the opposite. You can experiment to get a healthy starter with the flavor you like. When learning you should start with a proven recipe..