One line per command, printed in the shell. The detail lives in the manual pages; the shell’s own language — quoting, variables, if, for, scripts, $PS1 — is in man sh, and the filesystem it all happens in is man hier.

help <name> is the short version of one page without leaving the shell, which is what bash’s help does for a builtin.

Tab completes what makes sense where the caret is: commands and filenames in the first word, directories after cd, manual pages after man and apropos, files after head and tail, and what is inside a directory once its slash is typed. One match completes it, several extend as far as they agree, and a Tab that can add nothing shows the choices under the line — a screenful of them, and then a count of the rest, because the answer to a list too long to read is another letter.

A word beginning with - is a flag, and the flags a command has are the ones its manual page lists under OPTIONS — offered with the line that describes each, so ls - and grep - answer the question without leaving the prompt. There is one place a flag is written down, which is why Tab and man cannot come to disagree about what it does.