Starting from your first question:
Your answer did not give the interviewers any indication that you understand the technology challenges behind your automation suite. They were expecting answers that would cover possible technical solutions like:
- Changing from a full browser to a headless browser stack
- Optimizing code to reduce inefficiencies
- Removing static waits from code (e.g replacing
Thread.Sleep with a dynamic wait)
- Running multiple tests concurrently instead of a single-threaded run of all tests
- using distributed systems to farm and manage your test runs, up to and including container farms over the cloud
- Minimizing the number of times lengthy setup and tear down routines have to run in a suite (while keeping your tests independent)
- Regular refactoring and deprecating obsolete or never-failing tests
- Adjusting scheduling to run a light suite of must-have tests on a per-build basis and longer test suites overnight or on some other schedule
Any of these answers would have been more satisfactory to the interviewers than the one you gave. Your answer would have left the interviewers with the impression that you did not know how to improve your test suite's performance or run time.
Which leads into the second question: the interviewers wanted to know more about the architecture of your regression system. You mentioned in a comment on one of the answers that they wanted a drawing. This is because the simplest way to illustrate any kind of architecture is with a diagram.
They were expecting you to know such things as:
- Whether your build server worked on a continuous build, continuous deploy, scheduled, or on demand basis
- Whether your regression suite used distributed on local resources
- How individual test allocation within the suite was handled (e.g. all tests run on one client system, all tests run on the server, a random selection of tests run on each server)
- What design patterns you used in building your tests, such as page object patterns, and whether you have it all in one big project or you separate your page objects so they can be called from multiple separate test suites
There are many other things you could have covered, including the advantages and disadvantages of the software and hardware architecture your tests are using and the reasons for the decisions. That kind of answer would have indicated to the interviewers that you understood what you were doing with your automation.
The answer you did give to their first question implied that you didn't understand what you were doing, and your inability to answer or understand their second question probably confirmed that impression.
This might seem harsh, but this is how interviews work. The interviewers are looking for reasons not to hire you. In this interview, you gave them reasons.