They are being added somewhere in your CRM. They are most likely getting tagged from your flow too. You can always export your contacts and filter by whatever tag the ‘subscribe’ form uses and import that into Mailchimp.
To do this automatically will require a bit of Velo code. It’s possible, and the function you are looking for is called “wix-fetch”. Here is a link where someone did it a few years back: https://www.wix.com/velo/forum/community-discussion/integrating-mailchimp-with-wixcode-using-wix-fetch