Specifically, I would like to know how a land or sea based large island (Isle of Wight size-ish) size non sentient organism could be home to a society of much smaller people sized sentient organisms.
Most symbiotic relationships start out as parasitic ones. Then the parasite gets an additional advantage by making their host prosper.
Consider a floating living island. It is basically a bigger and more sturdy Portuguese man o' war. It has strong stingers both for fishing and for protection. At this point it is not very big, just a few meters.
Along comes a smart sapient. It figures out how to bypass the stingers. It may be as simple as sailing a canoe and never putting a hand in the water. Careful about that paddle, though.
In the beginning, it is just a place to sleep where the predators of the mainland can't reach them. There is wear and tear on the island, so it is definitely a parasitic relationship.
Then they start feeding the island. At first it just to help it regrow worn parts, then it is also to make it grow. And grow it does.
Now there is room for more people!
Every time the sapients go out fishing, they give part of the catch to the island. And the island grows bigger, and the stingers grow stronger.
The most dangerous predator is other sapients, so our islanders start enhancing the defenses so enemy canoes can't get through. Maybe just feeding the stingers until they can upend a canoe, maybe something more creative. Only natives know the safe path through the stingers.
I have problems imagining a living island the size of Isle of Wright, but it should be possible. One possibility is that the living parts is just a fringe while the central parts is a wooden platform built by the sapients.
its possible humans have a pretty symbiotic relationship with dogs. If anything being intelligent opens up new way to be symbiotic because you can anticipate behavior better and come up with creative solutions. being useful to humans is one of the most successful strategies out there, especially if you more useful alive than dead.
A mobile home has all the benefits of a nomadic lifestyle without any of the drawbacks. The Isle of Wight is almost 150 sq miles. On something that size you can have full blown town with agriculture. The difficulty with getting on and off will create some natural defense as well. How much rain water they can collect will be the biggest limiting factor. No wells on a turtle.
This is not improbable. The case can be made by pointing out that the reverse situation has developed on planet Earth where a relatively large sapient species lives in facultative symbiosis with a small less sapient species. This is the relationship between humans and dogs. The evolution of companion animals living in co-existence with a sapient organism is something that could easily evolve throughout the universe especially when both species benefit from the relationship.
Basically dogs are stronger and faster, possess better senses of hearing, small and possibly sight, and are capable of defending their vulnerable "masters" from predators. Humans provide food, cooked food provides more energy than raw, and assist in raising of their cubs. Dogs have developed the ability to delegate problems they themselves cannot solve to their bipedal companions. Tests show wolves are better problem solvers than domesticated dogs. But they learned to let their humans take care of the hard problems.
There is no reason why a smaller sapient species and a larger less sapient organism could not form a facultative symbiosis. In fact, in the absence of a canine species on this planet, as a hypothetical alternative evolution, then humans might have formed a master and companion animal relationship with elephants.
The role of companion organisms and sapient species isn't often considered. If this relationship can arise here on Earth between humans and dogs where the less sapient species is the smaller, then there is no probable biological impediment to a similar relationship arising between a small sapient organism and its larger companion animal.
Dogs and humans can share the same habitats. A sapient and its larger companion creature will have somewhat different domestic arrangements. Instead of kennels or even sleeping in the same bedroom, "stables" or holding pens would be attached to houses to keep both species close. Wide streets and footpaths will be essential features of city landscapes.
Transportation might present problems. Marine vessels might accommodate elephant-sized companion animals, but few in number. Aircraft would be definitely difficult. However, ingenuity might find a way.
If you only have one small sentient individual per big organism then the small ones would evolve adapting their life cycles to the big ones, which means they would not herd in big numbers or mate often.
Maybe the big ones can produce very big litters but the small ones not so much, which would mean that there won't be as many small ones and lesser generational change than with humans. That would make their evolution slower. Also an individual protected by a big organism wouldn't need to develop ingenuity as much as humans did to avoid predators and find food.
I'd say the odds are slim.
The symbiotic relation between humans and cows has already been pointed out as a good example. If you are interested in a bigger less sentient organism, you can use walnut trees. If you ask specifically about animals, domestic elephants are a bigger animals actual example, and whales are a fictional example in Arthur C. Clarke's The Deep Range.
Food in Exchange For Protection
One of the forms of symbiosis is when one creature provides food while the other protection. Humans have this type of relationship with a number of domesticated animals including several which are larger than us. A classic example would be dairy cows. They might not be quite as tall as we are but an adult cows weigh in at 680-770 kg (1,500-1,700 pounds). They provide us with milk, we protect them from threats and take care of them.
Considering something like this has happened on our planet, I would say the odds of this happening on your planet is extremely likely. So you can have an intelligent species evolved with a similar large herd animal that provides milk or some other food while the intelligent species protects it from predators.