UNIX at the operating system level treats all files as sequences of bytes, without imposing or being aware of any more complicated data structures. When originally developed this was something of a departure from other operating systems which often had complicated record management facilities built-in.
User programs can therefore impose any structure they like, but other OS utilities can copy and manipulate files without having to worry about structure.
Many other UNIX facilities (hardware device access, pipes, network sockets) also have file-like interfaces. Not all use the open() system call to create or obtain access to the facility, but data is almost always transferred using the read() and write() system calls. (A later research operating system based on UNIX called "Plan 9" even tried to make all file-like resources accessible via UNIX-style pathnames and the open() system call.)