grep -r string .

The flag should go before the pattern.

-r to search recursively, . refers to the current directory.

Why use . instead of *? Because on it’s own, * will (typically) not match hidden files. See the last paragraph of the ‘Origin’ section of: https://en.m.wikipedia.org/wiki/Glob_(programming). Technically your ls command (lacking the -a) flag would also skip hidden files, but since your comment mentions finding the string in ‘any files,’ I figured hidden files should also be covered (the find commands listed would also find the hidden files).

EDIT: Should have mentioned that -R is also recursive, but will follow symlinks, where -r will ignore them.