Substack and GDPR

 Hi everyone!

I'd like to create a newsletter for my site, using another tool by third party and not connected to my site, except for some links for some content, called Substack.

For GDPR, how can I manage it?

Also if the go to this new tool to subscribe to the newletter, do I have insert it in the privacy policy?

Do I have manage the user data, or is it a job for Substack?

Many Thanks for all replies.

Kindly Regards.