It's a variety of techniques depending on the site. See https://github.com/magnolia1234-new/bypass-paywalls-chrome-c... and https://github.com/magnolia1234-new/bypass-paywalls-chrome-c...

Techniques include blocking cookies, specific JS scripts, redirecting to a different URL, changing the user-agent to pretend to be Google's indexer, changing the referer to make it appear the user is coming from a Google search, and removing or hiding elements from the page that get in the way of reading the article.