All domains that have been registered initially through 1&1 will use 1&1 Name Servers (DNS). For domains that are transferred to 1&1, the DNS settings can usually be chosen when initiating the transfer and therefore will vary.
Depending on what type of 1&1 package the domain is being added to (or ordered with), the DNS settings may slightly vary but all domains will use the 1&1 Name Servers (DNS) and 1&1 Mail Server. They will also point to the correct IP address if the domain is added to a package that offers web hosting (web space).
The DNS settings can always be altered after the domain has been registered to point to a different IP address, mail servers, or even change the DNS altogether (DNS Transfer or CNAME).
Domains that are being transferred to 1&1 from another company will prompt you to configure your DNS settings when initiating the transfer but can always be changed at a later time if necessary.