Yes both open and regular students are available for both iits and nit's but the eligibility criteria is student has atleast 75% marks in 12th board.

dear SKR ,

yes open inter students form NIOS (national institute for open schooling ) are eligible for applying jee main exam .and also eligible for taking addmission in different IITs and NITs according to their rank in jee main and jee advance exam. if you want to know more information about the the eligibility criteria of jeem main exam then you can go through the below link -

https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-eligibility-criteria/amp