You need a company to sponsor you through it, you can't just apply.
Once you have it, it can be transferred to a new company if you move, but it lapses after 12 months if you are somewhere that doesn't require it.
Almost! SC does require a sponsor, and that sponsor cannot just be any company. SC itself is clearance for you
in the specific role. So if you change jobs, your clearance will need to be transferred but essentially this is almost like a re-application. If you leave your job and have a period of not working, you should not bank on the "12 month" thing at all as behind the scenes it just doesn't work like that.
As for getting on the ladder - nobody should be stating that applicants must have SC (or DV or anything else) to
apply for a job. You aren't really supposed to disclose your clearance status to anyone, so how can a recruiter ask? The official bodies take a very dim view of that sort of recruitment practice and it is possible to report people who do behave like that. Usually, job recruiters refer to it as "must be eligible for SC" or something similar to that.
If a job role is out there and happens to need SC, just apply for the job like you would any other. Unless you've got a criminal record, have spent time in certain countries or have any of a few personal issues, you'll likely be just fine!
