For simplicity the guide is for setting up a stand alone server with node-red. Are you trying to host a web sited and node red on the same server. If so you need to get a better understanding of nginx.
I've never done what your asking because node-red does not like to have 2 seperate root '/' locations for admin and dashboard.
if you forward it with nginx it can get messy so for it to be easy you will have to pick
in your node-red's settings.js file you need to set your UI settings for dashboard to / and change node-red to a /admin
or keep admin at '/' and change your UI root location
then change nginx file to point to each accordingly
To be honest, the first config change I always make when installing Node-RED is to change the Editor to be on /red since I always want to keep the root url clear.
NGNX should have no issues with this. And Dashboard will be on /ui anyway by default. I usually put a menu of some sort on the root URL just so that there is always something useful there.
Node-RED does, of course have many different URL's and some nodes add their own. However, because Node-RED has its own PORT, you can easily configure a reverse proxy to redirect it to a different virtual web. This is important in production environments since many enterprise firewalls will block port 1880 by default.