Hello,
You can go for IITs for B.tech course after completion of your Diploma. For that, you need to appear for both JEE Mains and JEE Advanced. Even if you qualify JEE Mains with good ranks, you won't be able to get a seat in any NITs IIITs and GFTIs. You will have to appear JEE Advanced and then try to get a seat.
Also, in other engineering colleges, if you ta k admission through lateral entry after your Diploma you get direct admission into the second year of B.tech. But in IITs , you get admission into the first year of B.tech after your Diploma.
For more details please visit this link https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/how-get-admission-in-iits-after-diploma/amp&ved=2ahUKEwj4jOGEl_DqAhVq7HMBHazfBuEQFjABegQIERAI&usg=AOvVaw07Xi_tNw3CXWelXWsS2BF8&ampcf=1
Hope it helps

Hello,

Yes, you can get admission in IIT but you must have passed 3 year Diploma recognized by the All India Council for Technical Education (AICTE) or a State Board of Technical Education (SBTE). Admission is based on merit in JEE Advanced to appear in JEE Advanced you must qualify in JEE Main. And you will get  admission in first year B.E/B.tech.

Hello,This may help you

Yes, youcan. You'll need to qualify through JEE Main and JEE advanced and youcanget admission for first year of four year degree course. Many state or private colleges take admission ofdiplomaholder in second year directly but not theIITsor NITs.