16 DC16251, no RJ45, how to connect to router?

You can either use Wifi (assuming you have a wireless router) or if it must be wired, you'll need a USB to gigabit Ethernet adapter.  Example:

https://www.trendnet.com/products/usb-c-ethernet-adapter/usb-c-to-gigabit-ethernet-adapter-TUC-ETG-v2

1)  Does DELL sell that device or do I go to an outside vendor (AMAZON, TRENDNET, etc.)? 

2)  Does it connect to port labeled USB 3.2 Gen 2 (10 Gbps) Type C port (# 5 in the Manual)?

Dell probably does sell one, but it'll just be a relabeled adapter from someone else -- Dell does not make the hardware any more than it made the notebook itself.  

You can find these in both USB-C or USB-A versions -- either will work equivalently.

i would suggest u to reach out the customer services department, as they are quite efficient and helpful.  me as a student, i feel difficulty and dell user,i reach out to them only.