Admission to B.Sc courses in non-medical subjects depends on the the decision of various states & universities offering the respective courses. The release of application dates will vary in different colleges for different subjects. After the release of application, you can register yourself & apply for the particular subject of your interest through the official website of the college. Pay the application fees & fill up the date for completion of the process & wait for the counselling process.
For example, admission to BHU has begun from June, 2021. Last date to apply in Lucknow University is 20th July. Colleges affiliated to Calcutta University has started the application process in June & it is continuing till the entire month of July. So, you see it depends on the institution. You need to look out for the release of the applications by visiting the official website of the respective college.
Hope this helps.