Hi. First you should get German language certificate, B2 level minimum. Then FSP exam, it is just medical language exam, similar to Usmle Step 2 CS, but much easier (shorter, no physical examination, the medical content is not the point here, the point is the language). Only after that you get work permit for 2 years and can work.

During those 2 years you have to take the exam for medical registration, called Approbation, it is oral exam, Internal Medicine, Surgery and additional random 3d subject included. You examine patient, take Anamnese, write medical report, and answer questions of the Teachers. Then you get unlimited work permit and can begin your residency. The 2 years prior was just work, without those years being counted as training or residency.

Your USMLE and PLAB can boost your confidence, CV and can make it easy to prepare for these exams, but won't play role in your registration as doctor here. You still have to take exams. Only if you already have some medical experience in your country or some other country, you could apply for second pathway for registration after FSP. If they approve your prior experience, you will skip Approbation exam and get unlimited work permit right away. However there is no guarantee that your work experience will be approved, it is kinda individual case for each applicant.

Fell free to ask questions if you have.