Thanks! They write "sending hashed first party conversion data from your website in a privacy safe way" but it's just hashing (https://support.google.com/google-ads/answer/9888656) the email address etc, which really does not deserve that description.
https://support.google.com/google-ads/answer/10763826?sjid=1...
The help docs are a bit cryptic, but the UI within Google Ads is a lot more straightforward in how it works. It searches the landing page for an email address to automatically pair with a conversion event. All the advertiser needs to do is accept some privacy policies and enable the feature.