I am designing a monster that is as large as a continent. It flies across the planet very slowly and has basically a snake form. Life cannot exist on the planet's surface so the animals are drawn to the "snake".
What draws a ecosystem to its surrounding and how this happens? Can humans build houses and villages on him?
While we could talk about worlds on the back of a giant turtle and similar things, let's explore some other options with "continental scale creature" with its own ecosystem and villages on top.
Pando is a clonal colony of quaking aspen trees covering 100 acres estimated to be anywhere from 10,000 to 1 million years old. It's basically like grass, all the trees share a common root system.
One could imagine a continent dominated by a single clonal colony of giant trees. Animals and plants would live in this "forest". Humans could live in or among the trees, or harvest them.
Imagine a world with a vast ocean. There are large floating island chains. These chains are really ancient, continent sized leviathans. Each island in the chain is a part of it that sticks above water. They float just below the surface on ocean currents, filter feeding from the rich life just below the surface of the ocean. Over millennia, debris, flotsam, its own excretions, and mineral deposits have formed on its upper surface to create soil.
Humans who encounter these islands may not even know they're creatures until someone dives deep enough to get below it. Or until someone digs deep enough and agitates the giant.
SCP-169 has elements of this idea in modern times. Note that while that particular SCP entry is SFW, the SCP wiki gets pretty weird. Lots of great ideas in there, but click around at your own risk.
The good, the bad, and the ugly
Australia is basically 4,000 Km long and wide. I'm basically 1.75 meters tall and 0.3 meters thick. That's a ratio (as good as any) of 0.17, so I'm proposing that your creature is at least 680 Km thick (making it a tapeworm, not a snake).
Earth's atmosphere is 480 Km thick. But most of it (e.g. 50%+) is in the first 16 Km.
The FAA mandates a minimum of 1,000 ft above everything in a 2,000 ft radius. Granted, that's for airplanes, which are less than 1% of the size of this critter. (A lot less, actually, but let's roll with 1%.) That means your critter needs a minimum of 30.5 Km. That's not very much, considering it's 680 Km thick, a burp may cause it scuff the ground.
Final number: the top of the creature is 230.5 Km into space. That's inside the range of low orbit. The bottom line is your critter probably has a snorkle descending into the lower atmosphere through which it breathes, and typhoon-force winds happen around the opening of the snorkle. Heaven help the good folks underneath it when the critter sneezes.
That was the good, now let's talk about the bad
I haven't done the math, but I expect the mass of this creature given its proximity to the planet will cause tidal changes and tectonic shifts. The world will literally reach up to the creature. Probably only by feet (maybe a dozen or so), but it'll be enough to fracture fault lines, cause minor eruptions, change currents, and basically make life on the planet itself something of a hassle. You said in a comment that you didn't expect life to exist on the "floor," so this might not be as bad as it sounds.
Aaaaaand the ugly
The problem is that the creature won't have enough mass to generate significant gravity. Especially in comparison to the planet below it. Since the top of the creature is in low orbit, you can't build anything on its back unless you have dome technology and can mine oxygen from its skin (mining the creature would be big business, btw... humanity being less pestiferous than bacteria is to humanity). And you can't build on the bottom, either, since the planetary gravity will simply pull you to the ground.
You would, however, have cliff-dwelling birds making nests in every fold of skin or everywhere they can make a bit of mud stick.
Creatures of this size tend to be very impractical unless you're in the mood for full fantasy — in which case you simply declare it to be so and have fun. (I love Terry Pratchett's work, BTW.)
Every macroscopic plant and animal has an ecosystem of microscopic life living on and in them. You have billions of bacteria, viruses, and fungi living on and in you. Some animals including some people, have macroscopic multi celled parasites and pests living on and in them.
So a creature a lot smaller than your example would have an ecosystem living on and in it.
That was featured in the National Geographic speculative biology "Alien planet". Darwin IV is home to a gigantic creature known as the Groveback. As its name suggests, it has a grove of trees on its back!
The Groveback absorbs nutrients from the soil. The trees absorb the nutrients and generate sugars and complex molecules via photosynthesis. These, in part, feed the groveback. And, wherever there are trees, there are animals feeding on them, fungi feeding on leaf litter, and the list goes on. You have an ecosystem.
Now to your "snake": it is too big to fly via wings. Think of a lighter-than-air gas filling its interior. This gas may be breathable to some creatures, plants and other things living inside it. Now, make the upper body skin more translucent. The inner cavity has a breathable atmosphere (maybe Hydrogen) and receives light. It gets waste products from the giant creature and recycles it. (Think of corals and the algae living inside them. They recycle waste products to feed the corals). Some of its product feeds the creature. The creatures may evolve a way to venture and look for such a "snake" and dig its way inside. When two or more snakes congregate, hopping from one creature to the next is easy.
A side note: If life cannot exist on the planet, how do those snakes live?
