Question about API specifications

Hi, I'm trying to understand what features are there in pact and pactman, one thing I didn't find in the readme is how I can use a swagger/openapi3 spec to generate and use for contracts? Can I do this with pactman or is this part handled by pact broker etc?

Also I'm not sure I understand how the pact file works with it. Is it being generated when tests run to be used later?
Thank you!