If A defames B, the fact that the parties live and work in different countries is not a bar to a defamation lawsuit. This is not changed when platform C is headquartered in a third country. A choice of law question does arise, which is important because B can always sue A, but they might have a legally-better outcome if they sue in the defendant's jurisdiction (plaintiff-friendly rules) yet they might be concerned over the "home courts advantage". The EU has a rule applying "to non-contractual obligations in civil and commercial matters" which expressly excludes "non-contractual obligations arising out of violations of privacy and rights relating to personality, including defamation" (there is no uniform EU answer). This article touches on such considerations in transnational defamation.
Whether or not some specific content rises to the level of defamation depends on the actual facts (was there plagiarism, which is not a legally-defined category unlike copyright infringement), and the choice of law.