What is present in the liver by which ammonia converts it into urea?

Ammonia is converted to urea in the liver via urea cycle. this cycle takes place in the mitochondria of the hepatic cells. For further references check Lehninger's Principles of Biochemistry by Nelson and Cox.

The conversion of ammonia to urea takes place in liver and to some extent in kidney also known as urea cycle It is a step by step process catalysed by various enzymes at each step L-ornithine carbamoyl phosphate L-citrulline argininosuccinate fumarate L-arginine L-Asp L-aspartate CPS-1 carbamoyl

Ammonia is a toxic by-product of protein metabolism and is generally converted to less toxic substances after it is produced then excreted; mammals convert ammonia to urea. Urea is simpler in structure and less harmful for the body. Ammonia is an extremely toxic base and its accumulation in the body