It's hard to answer the original question, but I expect that they spend a lot of time looking for a site - then working out how best to fit a layout into the space available.
It would be interesting to work out why so many of the newest tracks seem to have gone down like a ton of s*......er..make that lead
Herman Tilke is an enthusiast like us, he races cars and has clearly tried to make good tracks - and I would also argue he has not fully succeeded.
So, why the failure? It's not simply because the original great circuits were street races modified and the new ones were "designed".
Spa - nothing like it's original and was, of course, designed to be the track it is today. Even Eau Rouge is no where near it's orginal shape for example.
Monza - was started off when racing was very different - to call it a temple of speed is quite appropriate. It's layout is not inspiring (imagine if Tilke came up with that one), but somehow it can and could generate great races. It's corners are very simple, but subtly different. The Monza of, say, 1967 demanded precision and courage from it's drivers. Other places might favour the more acrobatic........
Hockenheim - again built to different criteria (originally).
The Nordschleife was designed as a racing circuit (test facility) not as a public road. It has every combination of corner and elevation change (some are huge - Ex-Muhle).
One thread that seems to run through most of the great tracks is SPACE, the use the landscape and cars have room to breathe. The modern tracks seem to feel like they are crammed onto a postage stamp, with corners joined up by straights.
Another thing missing today is variety. Surely it is better that one week a Grand Prix could be run at somewhere like the original Monza or Hockenheim (all about speed) and another week they could be crammed into Brands Hatch or Monaco.
I reckon I could do a better job BTW..........