How do sites use other people sub domains to run their site through?

Not sure I am but thanks for the lead.

Say ABC Ltd runs an online web app of some sort.

123 Ltd sets up a sub domain and points it to a particular url on ABC ltd, who do something (not sure what)

Then a customer of 123 ltd, goes to the section called 'Some service or other'on 123's website but is actually using the app provided by ABC ltd. The customer of 123 never knows about ABC ltd, but think they are on the website of 123 Ltd, as all the urls are for 123's website.

Does that make sense?

I think so. So you'd want to setup a wildcard subdomain under ABC LTD with the subdomain 123 LTD. The server serves content from ABC so custers from 123 get everything ABC has to offer

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