The JEE-MAINS and ADVANCED brochures give extensive details of the selection procedure. After getting your score you will have a cut off score to write JEE advanced. Also after the state boards and CBSE give the 12thstandard marks the normalized score (refer the JEE MAIN booklet) will be declared where 12th standard board exam will also be factored in. (5 subjects) then a rank list will be prepared and now you can make selections and allocations will be made based on the rank list and your choice. This is round one and you have a choice to accept the offer or keep your name for second round or totally reject and stay away from the selection process. Depending on seats filled a second round and so on and so forth is conducted till seats are filled or a cutoff date is reached when the process ends.
Considering your mains marks and your class XII marks, using normalization formula, your rank is declared by July 1 week an internet board- CSAB (central seat allocation board) is opened your register yourself on CSAB and key in your order of preferences i.e. college and branch. You will be given an option to fill in up to 100 preferences.
After the last date of filling choices is over CSAB runs its first iteration of allotment. It starts with the first ranker and checks his/her preferences and allocates him/her a seat and removes this seat from the allocation pool. It then goes to the next ranker and allots a seat. Thus it runs up to the last ranker and allots any available seat. This allocation is declared to students and are given time to pay part of college fees and get their documents verified some of the students would not have paid the fees, as they already have conformed seats in IITs BITs IIITH, II Sc etc so again a second iteration is run from first ranker. Everyone will either retain their previous allotment or get an allotment higher up in their preference.
This way a third and a fourth round of allocation is completed still some students might drop out because they have got a better option elsewhere after the first round.
On last spot round is conducted and allocation is made for remarking seats of govt funded colleges. Also allocation to private institutions is made in this round.
After some days of result day, they ask you to fill your choices in the order according to your priority. Now let’s say you filled a, b to z choices. After result of this round you might get a college or not. Depending upon the seats available. Now suppose one of the people rejects his seat because he was getting better colleges in other entrance exam there is vacancy left. To fill these vacancies there is further round in which this person is removed from the list of people wanting JEE MAINS colleges and your positions is improved. So that possibility of you getting better colleges increases, and so on third round.