Getting used to the new naming conventions. i.e. Preferences -> Settings. I normally just press CMD + Comma so I didn't even notice they renamed it on App menus. I certainly know they changed System Preferences to Settings.
As to the shenanigans, they involve sneaky things like a hidden remote image single pixel whose job it is to let the senders know you read the email. Even rendering in preview would make a log entry to the web server with your IP address. Social media and Big Tech and marketing companies are connecting the dots from multiple sources of data to build profiles on people. In many cases, their database may know you better than you do yourself. They know what you like, they know how old you are, they know where you work, they know if you have a spouse or kids, etc. All remote images are rendered in HTML email much like a web browser every time that happens, they have a log entry. If you click a link, the link might contain a unique code sent back to the server and now they know the specific individual they emailed clicked the link.
It all went down hill when email started to allow HTML rendering. Before that it was text and only text. Even today, attachments are converted to text and converted back to a binary when you save it.