Most websites have their domain name DNS connected with whoever they bought the domain from. So for instance if you use GoDaddy for your domain name, they also host the DNS. Just to clarify, DNS is what maps your domain name (approbo.com) to an IP address that your computer/web browser uses to find the server your connecting to. There are DNS servers that your local computer connects with to resolve the websites you go to.

Preparing for a sale before-hand makes this process much quicker. The best option is to open a new domain register account for just the business/domain you want to sell. This means a new GoDaddy/NameCheap Account, or a new Google ID/Google Domains Account. What we typically recommend to sellers is to get a new google email address, then create a new domain holding account just for the business they are selling, and transfer the domain to that account.