There are four Ways of joining Indian Army after doing Graduation/Btech-

1)Combined Defence Service Exam CDS CDS Exam or Combined Defence Service Examination is an exam conducted twice every year by Union Service Commission(UPSC).

2)Technical Graduates Course (TGC) TGC entry also know as Technical Graduate Course Entry is a type of entry through which an engineering graduate(only male) can join the Indian Army as an officer.

3)Short Service Commission (SSC)-It gives you the option of joining the Indian Army and serving it as a Commissioned Officer for 5 years. Once your tenure is over, you are allowed to opt for a Permanent Commission.

4)University entry Scheme(UES) - University entry Scheme offers you enter in Indian Army as an engineer.This is the entry scheme for all Btech graduates.

Hope this helps!!!

Yes, If you are interested in Armed force after B.tech then you can go for TGC (Technical Graduate Course) Entry for which you can apply Twice in a year or
SSC-Tech Entry which also you can apply twice in a year.

TGC Eligibility

Education Qualification: You should have passed the requisite engineering degree or be in the final year of the engineering programme.

Age limit: Your age should be between 20 to 27 Years to apply for this programme.

SSC Tech Eligibility

Education Qualification: You should have passed the requisite engineering degree or be in the final year of the engineering programme.

Age limit: Your age should be between 20 to 27 Years to apply for this programme.

The minimum hight should be 157.5 cm for men & for woman the hight should be 152CM.

Good Luck!