if a user debug build comes with adb root access by default, does it contain the su binary? If an Android user debug build comes with adb root access by default, does it contain the su binary to achieve this?
If not, is it any different to rooting your phone?
No, adb does not require a su binary to get root permissions.
On a regular build it is just the other way around: adbd is started by the Android system itself with root permissions. Up to a certain point it uses these root permissions (open port and so on) and then adbd drops the root permissions.
On an Android user debug build comes with adb root access adbd does not drop the root permissions, hence every command executed via adb is executed also with root permissions.