How sales to CPQ opportunity Integration works

Content

Hi,

When we create a transaction from Sales cloud to CPQ, at first using SSO a new transaction is created and lands on CPQ. And then integration is called, for eg if we take Opportunity Integration using OOTB functionality a SOAP generator call(GET- uses opportunity_id in request) is made from CPQ to Sales and in Result Parser mapping between sales and CPQ is done where the CPQ attributes are assigned with sales values.

But in the first place how does CPQ get the opportunity_id and pass as SOAP request?

Can anyone explain so that I can understand the concept.