NAS and attaching a My Book drive to a router port

Don't know enough yet to answer my question.
NAS's are something new to me so I'm trying to get up to speed (no pun intended).
As I understand it a NAS would hang off of a router port, and have its own built in IP address so it can be accessed from the net or just use it locally (or both).

I have also been checking out new routers, and see that the Linksys WRT1900AC, and higher have a rear USB 3.0 port. Since I don't have any USB 3.0 ports on my desktop (yet) it got me thinking. (bad thing on my part)

Could I connect a My book to the router USB port, and would it be the same thing as a NAS ? I'm thinking not since it wouldn't have own IP address?

Would I get good speeds using the My Book connected to the router since I don't have a USB 3.0 on my desktop?

Lastly how good are speeds to a NAS verses connecting a drive to a USB 3.0 desktop port ?
My current router an old Linksys DIR-615 specs indicates port specs are 10/100 so upgrading my router to a router with 10/100/1000 should increase transfer rates to the NAS or My Book ??

P.S. You blokes across the pond have your in act together !

Most routers that have USB ports let you connect a hard drive and it shows as a shared folder so you can use it there is a fair chance the speed could be better than local but there are a lot of factors to the equation

Routers do have the ability to 'look like' a computer. Mine is a Gigabit Wireless with 4 Ethernet ports and 2 USB 2.0 ports. I haven't any need for the USB yet because of my setup I also have an 8 port Gigabit Switch [needed for 5 computers, workbench, 2 NAS Ethernet drives and 2 Ethernet printers (Inkjet and Laserjet). An advantage to the WDC MyCloud NAS drives is I can access files on them using versions of Windows, osMAC [formerly Mac OS X] and Linux computer whether Wired or Wireless. The computer part interprets the files as needed when accessed by any computer. The Wireless part of the Gigabit Router is 802.11b/g/n, am looking at the future in replacing with 802.11ac equipment as I now have 2 computers with that capability.

I will say one thing I visited the Newegg web site, and read all the reviews for the MY Cloud 4TB drive, and they aren't exactly glowing. Especially the latest (all the other are for 2016 or older) confused me about the subnet comments.
WD 4TB My Cloud Personal Network Attached Storage - NAS - WDBCTL0040HWT-NESN-Newegg.com

I'm not sure if that meant I would not be able access my data from within my own home with the first version,and that doesn't sound right to me. I have no way of knowing what version drive I would get it's all factory sealed.