Dear Allen, I give you a brief idea about the journey from the entrance exam till college allotment. Suppose you cleared jee mains exam after that you have the option to give jee advance exam and you get very good rank in jee advance. After that you can apply for IITs, NITs according to your jee advance rank list you have to fill counselling form which you have to mention your Branch preference which you want to opt, according to your rank and number of seat availability you get your college and branch after that you take admission in that college after completing the necessary steps like document verification fee etc etc.

All the best dear