It's not provided in Shopify. Especially for any reasonable price.
You would not require a sub-domain as each of your shop- domains are separately hosted. Yet they are managed from a single admin.
Individual PayPal accounts can be assigned.
Each visiting customer is separate and do not cross over to the other shops.
Think of running your project as a hotel and not a single family rental.
If you have 100 rooms, it could be 100 individual stores, yet they are all managed by the front desk and the backend management.
One account, with one single log in.