The obvious answer is to just get YouTube Premium, though there are plenty of free options to do instead.
This article offers browser, browser extension, 3rd party websites, and mobile Android apps as a possible solution to watch YouTube ad free.
Browsers
Through Chrome's recent update to manifest V3 it has been difficult to continue using browser extensions to block ads. Thus, it is recommended to leave Chrome and choose another browser.
Brave (both mobile and desktop)
The Brave browser has an integrated ad blocking feature, which also blocks YouTube ads.
LibreWolf (desktop only)
The LibreWolf browser is a hardened Firefox browser that comes preinstalled with the uBlock origin extension. This extension blocks the YouTube ads.
Firefox (both mobile and desktop)
Out of the box the Firefox does not block YouTube ads and the uBlock origin extension is required to be installed to get this feature.
DuckDuckGo (both mobile and desktop)
The DuckDuckGo browser comes with the Duck Player which is a front-end mask to watch YouTube ad free and without distractions.
3rd party websites
The 3rd party websites either provide a front-end mask or have a copy of the specific YouTube video.
Invidious
Invidious is an open-source front-end mask to watch YouTube ad free. There exist extensions for various browsers which automatically redirect YouTube links to Invidious e.g. Privacy Redirect.
You might not be fruitful with Invidious, since YouTube is actively blocking each instance that tries to circumvent YouTube ads.
Odysee
Some creators clone their videos to Odysee as a back up to their YouTube channel. Thus you will need to check if the creator or video exists on the platform.
YouTube recently put in the obstacle to make copying even harder, thus the pool of existing videos might decline.
PeerTube
PeerTube uses a similar idea as Odysee with copying videos.
There are lots of other platforms that employ this technique. With some basic web searching you should become fruitful.
Mobile Android
Clipious
The Clipious app is a dedicated Android app for Invidious instead of using a browser. The same issues as in Invidious persist.
NewPipe
The NewPipe app plays YouTube without ads while providing additional features that only YouTube Premium users would have.
Acknowledgements
Using any of the mentioned options goes against the official YouTube's Terms of Service.
Also you are potentially stealing some revenue from the video creator, thus if possible and willing from your side, hopefully you are supporting them in a different fashion.
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