— Command: insert-buffer from-buffer-or-name

This command inserts the entire accessible contents of from-buffer-or-name (which must exist) into the current buffer after point. It leaves the mark after the inserted text. The value is nil.

— Command: self-insert-command count

This command inserts the last character typed; it does so count times, before point, and returns nil. Most printing characters are bound to this command. In routine use, self-insert-command is the most frequently called function in Emacs, but programs rarely use it except to install it on a keymap.

In an interactive call, count is the numeric prefix argument.

Self-insertion translates the input character through translation-table-for-input. See Translation of Characters.

This command calls auto-fill-function whenever that is non-nil and the character inserted is in the table auto-fill-chars (see Auto Filling).

This command performs abbrev expansion if Abbrev mode is enabled and the inserted character does not have word-constituent syntax. (See Abbrevs, and Syntax Class Table.) It is also responsible for calling blink-paren-function when the inserted character has close parenthesis syntax (see Blinking).

The final thing this command does is to run the hook post-self-insert-hook. You could use this to automatically reindent text as it is typed, for example.

Do not try substituting your own definition of self-insert-command for the standard one. The editor command loop handles this function specially.

— Command: newline &optional number-of-newlines

This command inserts newlines into the current buffer before point. If number-of-newlines is supplied, that many newline characters are inserted.

This function calls auto-fill-function if the current column number is greater than the value of fill-column and number-of-newlines is nil. Typically what auto-fill-function does is insert a newline; thus, the overall result in this case is to insert two newlines at different places: one at point, and another earlier in the line. newline does not auto-fill if number-of-newlines is non-nil.

This command indents to the left margin if that is not zero. See Margins.

The value returned is nil. In an interactive call, count is the numeric prefix argument.

— Variable: overwrite-mode

This variable controls whether overwrite mode is in effect. The value should be overwrite-mode-textual, overwrite-mode-binary, or nil. overwrite-mode-textual specifies textual overwrite mode (treats newlines and tabs specially), and overwrite-mode-binary specifies binary overwrite mode (treats newlines and tabs like any other characters).