"at work" - at the place where you work. "I'm at work." "I'm at home." "I left my cellphone at work." "in the job" - I don't think I've ever heard this phrase used. "on the job" - currently working. "I'm on the job at the moment." "I'm at lunch; I'll be back on the job in ten minutes."
Yes, job is profession, and work can mean either "the place where someone w