When subscribing to the newsletter, I collect your name and email address. This is only used for sending you newsletters.
When paying for the newsletter, Stripe ends up with your payment details, and whatever other information you give Stripe. I don't use this for anything other than receiving payments from you.
My site itself does use analytics (currently Google Analytics), so that I can see how often people read each article which pages are slower to load, what people search in Google for to find my site, and things like that. I only use that data in aggregate to make my newsletter better.