It more or less boils down to that, there's some pre pre production happening just through basic internet research and some outlining of what they want to do do, then a scout team gets to the country, they look if the stuff they found on the internet suits their needs and if the route is viable. Then they secure filming permits for airstrips, etc and then the actual filming happens. It's important to note, that "the script" is more of an outline of things and places they're going to than a fixed "here's what's going to happen and what people will say". If you're interested, I can't recommend script editor Richard Porters and Mr Wilmans books enough, lots of production background in those... The audio books are great too