generate an array of user login date using update_user_meta();

Try this, notice how I set the 3rd value of get_user_meta() to “true”

function user_last_login_list( $user_login, $user ) {
    // If last_login user meta exist
    if(metadata_exists( 'user', $user->ID, 'last_login' )){
        // Get all login dates as a single array
        $UserLoginDate = get_user_meta($user->ID, 'last_login', true);
    } else {
        // Initialize as an empty array if it's the first time user logs on
        $UserLoginDate = [];
    }

    // Add the current day to array of login dates
    $UserLoginDate[] = time();

    // Update user meta with the updated array
    update_user_meta( $user->ID, 'last_login', $UserLoginDate );
}
add_action( 'wp_login', 'user_last_login_list', 10, 2 );