Tampermonkey: YouTube Auto Skip Ads. Install Tampermonkey https://www.tampermonkey.net/ and add the script. Done.

youtube-auto-skip-ads.js

// ==UserScript==
// @name         YouTube Auto Skip Ads
// @description  Skips ads on YouTube
// @namespace    https://pirateboy.net
// @version      1.1
// @author       Jonk
// @match        https://www.youtube.com/*
// @grant        none
// ==/UserScript==

(function () {
    'use strict';

    function skipAds() {
        const video = document.querySelector('video');

        if (video && document.querySelector('.ad-showing')) {
            video.currentTime = video.duration || 9999;
        }

        const skipBtn = document.querySelector('.ytp-ad-skip-button, .ytp-ad-skip-button-modern');
        if (skipBtn) skipBtn.click();
    }

    const observer = new MutationObserver(skipAds);
    observer.observe(document.body, { childList: true, subtree: true });

})();