Hi J_D, welcome to AllStar.
Once your URI arrives, that is basically your radio interface into your Node. Your Node is also essentially your Server. Both Node and Server are configured (some aspects) from the Allstar Portal, which I think you’re familiar with.
The URI (as well as other radio interfaces, including homebrew ones you can even make yourself) takes signals from the radio and make them available to your PC.
Depending on the make/model of your radio, the URI->radio connection options may vary.
More of the radio inter-connection info can be found here => https://wiki.allstarlink.org/wiki/Radio_Connections
Once your radio is connected and available on your Node/Server, others can dial your node number and connect their Node to yours… e.g. *3 12345 where 12345 is the node # etc.
More of the info on Node connection/disconnection can be found here => https://wiki.allstarlink.org/wiki/AllStar_Link_Standard_Commands
Active (online) Nodes are show on the Stats page => http://stats.allstarlink.org/
Nodes keyed-down right “now” are even shown here => http://stats.allstarlink.org/maps/allstarNodesMapKey.php
Everything runs on top of Asterisk and app_rpt. Allmon2, for example, connects to a manager port running underneath the Asterisk running on your node. Not sure of a good Wiki read for this one, but the Community board may have some posts here and there that get more into how the backend works.