Short version: nothing, unless you choose to sign in. Every lesson is free to read without an account.
Reading the site does not create an account and does not set any analytics or advertising cookies. The pages load fonts from Google Fonts, and your browser contacts Google's servers to fetch them, which means Google receives your IP address as part of that request.
Sign-in is handled by Google Firebase Authentication. When you sign in, Firebase stores your name, email address, profile picture link, a user ID and the dates you signed in. Your browser also keeps a sign-in token so you stay signed in between visits.
The only purpose is to know who has asked to hear about new work. There is no mailing list running yet. Nothing is sold, shared with advertisers or used for anything else. If a mailing list starts, this page will say so before anything is sent.
Firebase is operated by Google, so the data is processed on Google's servers, which may be outside your country.
Sign out at any time from the button in the top bar. To have your account record deleted, send a message on LinkedIn asking for it, and it will be removed from Firebase.