There is no default ubuntu user in more recent releases…this gets added at first login. SSH cannot work if there is no account.
In recovery mode the micro-B USB cable shows up on the host PC as a custom USB device. The flash software has a “driver package”, and this is what understands and operates on that custom USB device.
When flashed with default software and booted the TX2 micro-USB port switches function depending on whether the connector to this is type micro-A or type micro-B.
A regular USB device (such as a keyboard or mouse) uses a type-A cable, and the TX2 would work with such devices normally for that type-A connector.
If the micro-B connector is used, then the detect pin switches the port to device mode. This means the TX2 itself becomes a device, and it is expected that the other end is a host which can use that device.
In fully booted device mode the basic programming can be customized, and one of those customizations is that the TX2 will appear as a USB ethernet adapter to the host PC. The TX2 gets assigned a 192.168.55.1 IP address, and via DHCP, the other end (the PC) listens to that DHCP request and assigns address 192.168.55.100 to the host PC. Any ethernet route to 192.168.55.1 will go to the TX2, and any route to 192.168.55.100 is essentially loopback from the PC to itself. This is a virtual wired ethernet. This is wired since it does not use WiFi, and it is virtual since the TX2 is only pretending to be ethernet hardware.
Adding additional packages, e.g., CUDA, is done via ssh over wired ethernet and ssh. The host PC running SDKM does not care if the wired ethernet is virtual or actual ethernet hardware.
One catch is that sometimes the host PC might be set up to not use random USB devices (as a security setting), and might need to be told to use that address.
Another option, if you know the TX2’s actual wired ethernet address, is to tell SDKM to use that IP address.
NetworkManager determines rules related to what to change when WiFi goes up or down, and is one of the more frustrating aspects of any networking setup. Sometimes WiFi going up causes wired interfaces to be dropped, but this is unrelated to the Jetson hardware and is purely an “Ubuntu thing”. The default flash has no ability to set up WiFi for you, and thus all package additions are coded to only work via wired ethernet. However, in the same way that you could tell the TX2 to use the true hardware wired ethernet address instead of the virtual wired ethernet address, you could tell SDKM to use the WiFi address. The trick is that you must manually set this up yourself since WiFi router and other considerations are beyond the scope of the TX2 (routers are secured independent devices).