ITAR doesn't have a clearance process with extensive background investigations. It is the lowest level of control for sensitive development operations. Non-citizens can work on it but there are paperwork hurdles that some employers don't want to bother with. It does also need to be conducted in US territory.
ITAR pertains to export/import. I'm not aware of person-specific clearances. In any case it would depend on the specific component or task being worked on.
there are not person-specific clearances, but there are person-specific non-clearances; e.g. there are lists of people and companies that you cannot do business with because of them being non-compliant with ITAR.