Dear aspirant,

For admission to IIT B.Arch program, you need to go through this:-

Appear for JEE Mains B.Tech exam. Clear the exam and qualify for appearing for JEE Advanced.
Appear for JEE Advanced B.Tech exam and get qualified for admission to B.Tech in IIT by scoring an AIR.
Appear for an AAT ( Architecture Aptitude Test) conducted by IITs. Clear the exam and get admission to IIT architecture degree.
(Additionally i just want to convey one thing out of this answer. If you are a science student and are running for B.Tech preps. You aspire for IITs and are thinking if you cant make it by chance for B.tech why not go for IIT B.Arch , you will have an IIT Brand in your degree.

Rethink, IITs are not as prestigious in Architecture as hype they are in B.Tech. Architecture is way too different from B.Tech and so are its colleges. There may be state or other national colleges in Architecture that are way too ahead of IITs. Search and explore on internet about same.) Best Luck.

Hopefully this will help you.
Thank you.