functions.php
file is a theme component, and it is auto-loaded by WordPress Core when the theme boots.
Although Woocommerce, or some other plugin, may have a file called functions.php, but this is not needed, because it is not the functions.php usually cited here and elsewhere, where people consider applying functionality to that file, then a file with that name used for a plugin may not be auto-loaded for default but may have to be loaded with plugin content
Woocommerce themes, which probably have a functions.php, though functions.php is not a required theme file.