How to implement a user registration system with partial confirmation?

Question

User identification

For example, on Bookmate there is such a system: they kind of send an email with a button to complete the registration, but at the same time, without completing it, you can make purchases. I mean, it doesn't really affect anything. + I wonder how they deal with bots. They first send a GET request to check the existing email in the database, if not, they immediately register and let them on the site.
I wonder how it's implemented