Include style.css in the Child Theme with PHP

You posted in a comment that the current theme uses the following code in its header.php

<link rel="stylesheet" href="https://wordpress.stackexchange.com/questions/318269/<?php bloginfo("template_url'); ?>/style.css" type="text/css" media="screen"/>

This is not WordPress best practice and hence the linked tutorials fail you.

How to go on from this?

Copy header.php from parent to child theme, remove the line containing style.css, then you can place the following code in your child theme’s functions.php:

function my_theme_enqueue_styles() {
    wp_register_style( 'parent-style', get_template_directory_uri() . '/style.css' );
    wp_enqueue_style( 'child-style',
        get_stylesheet_directory_uri() . '/style.css',
        array( 'parent-style' ),
        wp_get_theme()->get('Version')
    );
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );