Hi Stephani,
As official doc said:
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/set-up-subscription-center#test-your-subscription-center
The only way most people will be able to open the subscription center is by selecting a link sent to them in a marketing email from a customer journey, and links such as these include an ID that lets the subscription center know which contact has requested the page.
Thus for Marketing list, contact record will be added into or removed from a Marketing list when he subscribes or unsubscribes, there is no need to make operation manually.
You can add or remove any contacts of segment depend on your demand, but it takes some time to refresh membership in segment.
In summary, there is no association between contacts in segment and contacts in marketing list if you add or remove any contact record from them.
Here is process to setup a standard Monthly Newsletter:
1. Create Marketing Lists in the Survey app
2. Create a Subscription Form and add Maketing Lists into it
3. Create a Subscription Center and add Subscription Form into it
4. Create a Content Settings to identify your Subscription Center
5. Create a Customer Journey to test(You must open the General tab, then set Content Settings field with you previous Content Settings)
6. You contact(he has already existed in segment) open the subscription link from email
7. He checks Monthly Newsletter checkbox
8. The contact will be added as a member in the selected Monthly Newsletter Marketing List
9. If he opened link from email again and uncheck Monthly Newsletter checkbox, its record will be removed in Monthly Newsletter Marketing List
You could read tutorial below for a full Subscription center process.
https://blogs.technet.microsoft.com/lystavlen/2018/04/23/subscription-center/
Regards,
Clofly