Hello!
To become a scientist DRDO and other research PSUs you have to first get a BE/B.Tech. degree. The ways of getting employed by DRDO as a scientist, are -
For more details please refer to this site
https://www.google.com/amp/s/engineering.careers360.com/articles/isro-drdo-barc-dae-how-work-here/amp
After completing you BTech/BE/B.Sc., you can apply for ISRO but the thing is ISRO advertises it's jobs vacancies on its official website for scienctist and many other posts, so follow their link to stay updated. You have to also appear for ISRO exam and syllabus will be available on their site.
Thank You
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