The aim of the article is to summarize the author's work, experience and publications in the form of a presentation of a coherent concept (requirements) for creating digital twins of entire factories. The article draws attention to the diversity of definitions of the concept of a digital twin. However, the main emphasis was placed on the factory's functions and its structure. The factory components were distinguished into local (technology, machines, devices) and global (material flow and control) components. The concepts of digital twins have been defined in relation to them. The article emphasizes the need to use simulation and poses challenges related to the use of simulation in digital twins of factories. Available simulation tools based on DES (Discrete Event Simulation) are discussed, as well as the challenges faced by users of DES applications and the doubts related to them. The need to define a new way of thinking related to simulation modeling, while taking into account the benefits of DES, is defined. This paper presents this way of thinking as a paradigm shift in simulation modeling for building digital twins of factories and shows its practical implications.