Dear Akshat,
For admission to IIT Kaharagpur in B.Arch programme, you need to appear in paper I of JEE Main and not paper II. Paper II of JEE Main is only for admission to NITs, IIITs and CFTIs. And yes, after you qualify in JEE Main paper I and are in the top 2,24,000 rankers of JEE Main (qualify for JEE Advanced), then you need to appear in JEE Advanced Paper I and Paper II (Physics, Chemistry and Mathematics). You need to sit for both the papers and qualify in both Paper I and Paper II of JEE Advanced. Remember, it is mandatory to take both Paper I and Paper II of JEE Advanced (Physics, Chemistry and Mathematics). However, this is not enough. Even if you qualify in both paper I and II of JEE Advanced, it does not guarantee admission to B.Arch programme in IIT Kharagpur.
After you qualify in Paper I and II of JEE Advanced, you have to register for JEE Advanced AAT, which is the entry test for admission to B.Arch programmes at IIT Kahragpur and IIT Roorkee. registrations for AAT will begin in June first or 2nd week, while the exam is expected to be held in second week of June. The result of JEE Advanced AAT will be announced in third week of June and if you qualify in AAT, then you have to register and participate in JoSAA counselling for admission to B.Arch programmes at IIT Kharagpur and IIT Roorkee.