This will give you details on url structure: https://en.wikipedia.org/wiki/URL
If you are interested in technical details, search for RFC 3986
In the specific case, https://idmsa.apple.com/IDMSWebAuth/signin?path=//browse?login=true&language=US-EN&instanceId=EN&appIdKey=529eb2b096d5a3d54162171f0f29ba797e602812660013123243e58bc7bedf56&rv=1
- https is the scheme (the protocol to be used for accessing the web server, securely
- :// is the separator before host (server) name : idmsa.apple.com
- /signin indicates the root path to the page on the server
- ? it is followed by a query to ask you for login
The rest of the url will be forwarded to the server once login is true
- login=true&language=US-EN&instanceId=EN&appIdKey=529eb2b096d5a3d54162171f0f29ba797e602812660013123243e58bc7bedf56&rv=1
- is the query
- it has sent you to a login request (login is true when OK) and then to the web page
- other elements of the query (separated by &) include language and other parameters to be interpretedby the server.
Hope that helps.
