Setting indent-tabs-mode to nil will insert spaces to fill the columns a tab would have filled, but it will not convert tabs to spaces. For that you need M-x untabify.

There is no Emacs equivalent of vi’s :set autoindent. Instead, Emacs references a variable, indent-line-function, to determine the function to call to handle indentation. Setting indent-line-function to indent-relative or indent-relative-maybe, for example, will replicate the previous line’s indentation, according to the function’s algorithm. Normally, you don’t set indent-line-function but rather leave that to a buffer’s mode.

For example, in an Elisp buffer, the mode is Emacs-Lisp and indent-line-function is set to list-indent-line. In a C++ file’s buffer, the mode is C++ and indent-line-function is set to c-indent-line. In a Fundamental mode buffer, indent-line-function is set to indent-line.

Automatic code formatter is a different thing entirely. Ctrl+Alt+\ is an auto code formatter. Your article caused me hours of confusion before I found out what the VI equivalent of :set ai was on emacs….

and thank you emacs so nice this auto indent