Woocommerce is a plugin. The functions.php file is a Theme component, which is auto-loaded by WordPress Core when the theme boots.
While Woocommerce, or any other plugin, may have a file named functions.php but such is not required and it is not the functions.php typically referenced here and elsewhere when people suggest adding code that file, and a file of that name used by a plugin won’t be auto-loaded by Core but would have to be loaded by plugin code.
There are Woocommerce themes, which probably have a functions.php, though functions.php is not a required theme file.