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 -

• Direct ntry through Scientist Entry Test (SET) - 1st Class BE/B.Tech. in Chemical/ Computer Science/ Electrical/ Electronics & Communication/ Mechanical Engineering. It is a all India Entrance Exam conducted once in a year, duration is 3 hours.
• Campus Selection - Committee visits IITs, IISc, NITs, central universities and other selected institutions. Final year or Pre- Final year students of BE/BTech/B.Sc (Engineering) who have cleared all papers and secured First Class or equivalent CGPA (with minimum 65%marks or 7.25 CGPA)
• Registration of Students with Scholastic Aptitude (ROSSA) - It is for fresh Ph.D. scholars and those who have passed engineering/medical exam at graduate level
• Fellowship for instance : Aeronautical Research and Development Board Fellowship - For BE/B.Tech or ME/M.Tech candidates of Aeronautical Engineering

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