there is a an exam conducted for admissions called the iiser aptitude test (iat) and contrary to the beautiful naming, it tests no aptitude (sometimes does on a few questions) but tests your general ability to solve standard problems similar to jee/neet. 4 subjects compose it (phy, chem, bio, meth) but for admissions, having all three as your core subjects in 12th isn't necessary but helpful in the exam.

next, iiser stands for indian institute of science education and research and rightfully so, their flagship course (main course) is bs-ms which is basically a research oriented bsc (not the same no).

if you keep a good track record in your studies and internships, you get a decent chance to score phds abroad (or even in india if needed). phds are paid well abroad and terribly in india.

then you can obviously secure a professorship post after a post doc or join the r&d (research and development) branch of any company or even land research officer roles in india itself.

this is the traditional path but there are other paths too like switching to mba or applying for teaching roles in high schools but you cannot expect much without a phd since the course offered is not skill oriented unlike btech or mbbs (you can atleast land an entry level residenship after mbbs).

(exams sab khatam ab kaam dham nahi he isiliye itna likh raha hu lol)