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To open a file is to find or visit it. The two are synonymous. If the file does not exist, Emacs will open an empty buffer associated with that filename. If the file exists, Emacs will place the contents into a buffer associated with that filename.
When you visit a file, Emacs will pick a major mode. Emacs supports an array of detection mechanisms that can all be changed to suit your needs. They are listed here in the order they are applied.