wp_enqueue_style doesn't work but wp_enqueue_scripts works

Question

I have a code like this

function fs_express_theme_styles() {
    wp_enqueue_style( 'fs-express-theme-style', get_stylesheet_uri(), array(), filemtime(get_stylesheet_uri()) );
}
add_action( 'wp_enqueue_style', 'fs_express_theme_styles' );

that doesn't work. I tried different versioning and no versioning at all. As a result I don't get any errors in the console or debug files but styles are not loaded. However, this worked:

function fs_express_theme_scripts() {
    wp_enqueue_style( 'fs-express-theme-style', get_stylesheet_uri(), array(), filemtime(get_stylesheet_uri()) );
}
add_action( 'wp_enqueue_scripts', 'fs_express_theme_scripts' );

I am not sure why add_action( 'wp_enqueue_style', 'fs_express_theme_styles' ) didn't work but add_action( 'wp_enqueue_scripts', 'fs_express_theme_scripts' ) worked.

Can someone, please, explain?

At the beggining I tried putting everything in two separate functions like this:

    // We define the function:
    function MYTHEME_scripts() {
    wp_enqueue_script('jquery-ui-datepicker');
    }

    // Add the functions to WP loading list.
    add_action( 'wp_enqueue_scripts', 'MYTHEME_scripts' );

    function MYTHEME_styles() {
    wp_enqueue_style('jquery-ui-css', '//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css');
    wp_enqueue_style('jquery-ui-css', '//ajax.googleapis.com/ajax/libs/jqueryui/1.8.2/themes/smoothness/jquery-ui.css');
    }

    // Add the functions to WP loading list.
    add_action( 'wp_enqueue_style', 'MYTHEME_styles' );

that was taken from here https://wordpress.stackexchange.com/questions/137104/wp-enqueue-script-was-called-incorrectly, but it didn't work so while testing I figured out that loading styles through wp_enqueue_scripts worked

I also added this

$myfile = fopen("newfile.txt", "w") or die("Unable to open file!");
    $txt = get_stylesheet_uri();
    fwrite($myfile, $txt);
    fclose($myfile);

to both functions to test things out and I see that add_action( 'wp_enqueue_style', 'fs_express_theme_styles' ) never worked but add_action( 'wp_enqueue_scripts', 'fs_express_theme_scripts' ) did