Hello, 

Admissions for B. Tech programs for IIITs will definitely follow JoSAA. You can have your counselling for IIITs through JoSAA once they start it. 

And as of open admissions, they for other categories like DASA(for students abroad) etc. 

So, don't worry, if you want to apply on basis of Jee Mains score,  then wait for JoSAA. 

It will start soon after the results of Jee Advanced. 

Hope this will help. 

All the best for future.