Others more knowledgeable can chime in but clearances are good for a certain period of time and in the context of the person’s responsibility. Just because you have TS clearance, doesn’t mean you can see TS material outside of your responsibilities.
Once you’ve left the government, the only way you keep a clearance is via a defense\IC contract. That’s the theory. It unfortunately has been winked at way way too long. Many contractors carry way more cleared personnel then they need to do the job.
Exactly think of the TS as a building. Inside the building are many rooms - compartments, special accesses, etc. Getting a TS just gets you in the building, it doesn’t mean you can go into every room. The job you’re assigned gives you a ‘Need-to-Know’ which may get you into several rooms but you’re not going to know them all. The DOD, DOE, State, Fed LEO & IC manage their TS (with some small differences in management) and who gets into what rooms and why. There is cross organizational cooperation on this as the specific job may require.