is the most frequently used command for opening files in Emacs.
C-x C-f invokes the find-file command, which then accepts a file name typed in the mini buffer area. On RET key, Emacs visits the file, which entails creating a buffer, copying the contents of the file into the buffer, formatting the contents according to the relevant settings and mode of the file, and then displaying the buffer for editing.
Emacs has dozens of other commands related to the find-file command that can be used for more specialized needs. For example, find-file-read-only opens the file for viewing only. Use the find-file tag for questions pertaining to all such file commands.
