www.thirdslot.com/test-proxy/account.html

Same domain as the previous page — different machine.

Customer account

This page is your application.

Your existing system, untouched. The files were never copied, the code was never modified. The front-end server simply passes the request through.

SERVER 02 Application host
Hostname
app.thirdslot.com
IP address
144.126.131.181
Software
Apache — your application
Changes made
None

Who answers what

/→ Server 01 · 194.163.153.82
/test-proxy/*→ Server 02 · 144.126.131.181

The file you are reading lives on 144.126.131.181, but the browser shows www.thirdslot.com. That is what makes login sessions, cookies and callbacks keep working: the URL never changes.