How to control the level of water in the toilet bowl?
If the level in the tank is also extremely low it could be that the fill valve is adjusted way too low, thus not allowing enough water into the bowl from the refill tube.

The level in the bowl has nothing to do with the plumbing, it has to do with the design of the bowl. The track of the drain has an up "trap" and then down to the drain, so the water will only raise as high as the up trap then spill over into the drain. 

Here is a visual: