Hi, Orion connects primarily to the AD controller, which belongs to the domain, of which the Main server (or ad. web server, because the authentication is done by webserver) is member. To access other domains, the trust between those domains must be established.
For the physical server, if you run 'whoami' from the command line, you'll see your username, which will be proceeded by the domain name. For example "yourdomain\yourusername". That will tell you what domain your SolarWinds server is connected too, or rather what domain your user account belongs too. Generally, these will be the same unless your organisation has decided to create a separate domain for the monitoring platform to live on.