Cooking need not be equated to the activities done in a science lab. Cooking is related to taste, appearance and usefulness of the ingredients to us.
So cooking is mostly about experience, tradition, taste and some stimulus or appeal factors like colour and smell. These may vary from person to person also.
However, there are some general tips which are seen practised in the kitchen.
1. Leafy vegetables and many green vegetables have some water content in them if they are fresh. Cooking them do not need pouring of more water if you want them to be solid or semi solid. From experience, we may not add even a drop of water, or sprinkle a few drops or for some items add very little water. Some vegetables can be cooked in steam also.
Some vegetables can be cooked under pressure in cookers.
If we consume a variety of vegetables along with other items like grains, fats, proteins etc in whatever source, and consume sufficient quantities we get needed nutrients usually. So we need not much bother about the way we cook unless we totally vary from the traditional and time tested methods we are familiar and habituated.
However, the prime point is we have to cook hygienically, after cleaning the vegetables and physically ensuring that there are no pests or insects, and the item is not decayed or rotten. It should be washed thoroughly before cooking.
It is good and preferable if we can get vegetables grown without any chemical manure or chemical pesticides. However, as it is not much practical we have to take precautions of repeated washing, removing the skin and shells etc.
Cooking time also varies depending on how you want the cooked item-mashed, coarse, liquefied, solid,semi-solid etc. It also depends on the method of cooking and ovens. It needs experience by trial and error and customisation.
Generally, if vegetables are cooked in a closed vessel and at optimum temperature and pressure, and the cooking water is not thrown out, nutrients will not be lost.
Cooking, after all, is not just for nutrition or filling stomach, it is for the tongue taste also.