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.

Indeed, one of my biggest career mistakes was not taking a job that would have included a Q clearance back before 9/11, when they were handed out much more quickly and freely.

"an industrial or commercial contractor, licensee, certificate holder, or grantee of an agency, including all subcontractors; a personal services contractor; or any other category of person who acts for or on behalf of an agency as determined by the appropriate agency head."

Under the purview doesn't mean that you are employed directly by the US Government. Again, from the example, Snowden had a clearance from his previous employment. Yeah, it had to get transferred but he didn't have to get re-cleared all over again.