Security clearance does not stay with the person once they leave the job unless their new employer is (or is sponsored by) the same granting agency (i.e. DOD or DOE) and even then there is paperwork involved to set up the transfer.
You are correct. The clearance comes in the form of a designated position with sponsored need to access certain cleared information. When you are no longer in that job you no longer have the need to access, and hence no active clearance.
When you start in a new job, if it is a security clearance designated position, your clearance can be reactivated. If there has been a gap between old and new your eligibility for a clearance will have to be re-evaluated, also when changing employers. (You could have been fired or left under a cloud, a significant marker for security concerns.)
All clearances require a government sponsor. So tech companies can't maintain someone with a clearance unless they are working on a govt program. IC may be a little more flexible on that.
