Fortunately, you need not draw diagram in any interview. 
Explain in words only.
Like MS Word, qtp is a tool. It don't change depends on the 
Application.
I suggest that you better take training in Testing.

Hai friend QTP followed the COM architecture that means it's
based on object based program  like  follow the classes
,object ..
  ok bye

Architecture of qtp:qtp contains 5 sections i.e,
1.TOOLBAR
2.TESTPANE
3.DATA TABLE
4.ACTIVE SCREEN
5.DEBUG VIEWER

1.TOOLBAR:-
     It contains icons to perform operation on qtp.Example 
record,run,stop icons etc.

2.TEST PANE:-
       here we can generate the test scripts based on 
requirements.it contains 2 tabs.they are KEYWORD 
VIEW,EXPERT VIEW.

KEYWORD VIEW:-
     here we generate the test script in simple 
understandable language interms of item, operation, value, 
documentation.

EXPERT VIEW:- 
      Here we cangenerate script in VB script.

3.DATA TABLE:-
      Here we can store test data to parameterise the test 
script.
   It has 2 sheets. They are Global sheet,Action sheet.

Global Sheet:-
    when we have test data in this sheet,we can 
parameterize multiple action test scripts

Action Sheet:-
     when we have the test data in this sheet by using that 
test data we parameterize specific test script only.

4.Active Screen:-
     Itprovides the snap shot of application for each 
record and stmt.  

5.Debug Viewer:-
  it is used to view,set or modify the current values of 
variables in the test when a test stops at break point.

QTP is test automation tool n product of mercury/hp.
here we need to generate some script(through record option, 
OR step generator,OR discriptive prog)n same time according 
to requiremt we do some modefication . n check the impect 
of that action .