> Very basic question: [not basic at all question which has been the subject of decades of research and produced several specialized programming models]

(Brackets my own of course.)

Sharing data in concurrent programs is not trivial, especially in environments where data is mutable. The most trivial answer to the question is “message passing”, as in the SmallTalk notion of OOP or the Erlang/OTP Actor Model. Some solutions look much more like working with a database (Software Transactional Memory). Some models that seem entirely designed for a different problem space are also compelling (various state models common in UI and games like reactivity and Entity Component systems).