MQTT and Node red newbie question

I’m totally new to MQTT. I have a little experience with Node Red. I followed Lewis’ youtube on installing Mosquitto. I made a new user mqtt-user.
I did the initial test as he shows at the end of the youtube. I would like to use MQTT nodes in node red. I guess I don’t understand a basic concept. Do I log into HA as mqtt-user? How do I know what the server is for setting up the node? Is it a node red server or an ip address for the Mosquitto. It probably is very basic, but I don’t understand the concept.
TIA for any help.

I do not know what youtube video you followed, but I am assuming you have a mqtt broker set up. When you set up the mqtt node there is a field for a server and a button to create one. You need to put the ip address of the mqtt broker. If you are using HAOS and the mqtt add-on, then the address is the same as your HA address. Then go to the ‘security’ tab and put in the user name and password. It is probably a good idea to create a user specifically for node red.