How do people create unofficial api's (which provide json info) for big websites such as imdb? -

such api here: http://www.omdbapi.com/

do parse whole website's html , save fields on own database?

what design, programming-wise?

my simple java developer mentality says this:

1 - use jsoup (or other html parsing library) , save data frequently.

2 - create restful services return json, such "searchbymoviename()", "searchbyactor"

3 - make services public

is simple that?

it can be, yes.

you can fetch pages , scrape data in real-time (as people call api). it'll little slower there's less overhead , don't need worry stale data.