The difference between green tea and black and oolong teas is that with green tea, there is no, or very little, oxidation.
After the leaves are withered and their moisture content reduced, the leaf enzyme that makes the leaves change colour is destroyed by heating with either steam or dry heat.
Japanese green teas are steamed, while the withered leaf for China green teas is briefly roasted or pressed in a large wok.
As steaming is almost instant in killing the enzyme, Japanese teas typically have a very green liquor. While the wok roasting method for China green teas allows a small amount of oxidation and results in more yellow coloured liquors.
After heat treatment the tea manufacturing process for green teas is similar to that described for black teas.