As far as I know, most of them are non-tech's and the people in JIT are almost exclusively from the armed forces and there's some preference for PAF personnel when it comes to tech as well (notable exceptions can be there of course).
For civilians, it's a dead-end job as none of them climb much higher up the ladder and by BPS-19 or so, they're transferred to the IB and here and there.
And the test is one joke. Read
here for the questions that came up in the test conducted for the recent batch that was advertised.
This is not a federal force, rather a provincial one run by respective home departments as an extension of the police. They are commonly called "CIA Police" and are operational in KP, Punjab and Balochistan with their role quite diminished in Sindh since the CID and AVCC were improved.
Pakistan has no direct equivalent of the FBI. We have duties and jurisdictions spread over a number of inquisitions with extensive cross-jurisdictions as well.