Please explain differences between AFM and FCOM manuals?
The simple answer is that the AFM is a certification document whose content (procedures, limitations, and performance data) is fundamental to the airplane's certification requirements and is approved (both original and all revisions) by the regulating authority that certified the aircraft. You could say that the AFM is part of the airplane's type certificate.

The FCOM/AOM/Operating Data manual is just a company publication. Its content is not approved. It takes AFM procedures and expands them into a kind of company-recommended procedural framework, provides descriptive information, and provides non-certification related performance data used for planning, like cruise data, optimal profiles etc.