What I am asking is not if there is an possibility of an island (Isle of Wight size because it is an island I am familiar with) size tortoise , I will find a way around that, I just want to know if a human could build and live on an island size tortoise and what technology would be required. The tortoise to clarify would in my mind be walking through oceans rather than on land and is surprisingly enough not a turtle.
The biggest limiting factor will be access to fresh water. your only option is to catch rain water, which will be limited by environment and storage. exposed standing water will quickly be fouled by birds.
Your second biggest limit will be lack of wood, which tends to kill island settlements. without wood you have few tools, no fuel for cooking, little building material, and most importantly no boats. unless you have special trees that somehow grow on the shell any attempt to live on the shell is doomed.
topsoil is not going to stay on the shell with all the movement, not unless you have some kind of special vegetation to hold onto it (back to the trees problem). you will probably have to build enclosed soil beds for farming If they farm. Which again will be affected by available water and wood.
Building will require some extra bracing since they must support themselves and deal with motion but small wood buildings can handle that fairly well. You should be able to drill shallow holes and sink a shallow piton for anchorage.
on land they could jump off and find water and wood, in the ocean they are pretty much doomed.
Humans need water, food, tools, and fuel (fire), everything else is just bonus. But the more technologically advanced you want them the more resources on the island they need.
So you need a specially adapted ground cover to help hold onto soil (easily hand waved) and some kind of shallow broad rooted tree there are many of those. With that you might be able to build on the shell, and with the size it might just be able to have shallow streams or ponds, but they will not last long after rain without soil to hold water. The more jagged and uneven the shell the better for this. but your people will still need to catch as much water as possible and probably actively farm lumber.
Various forms of bamboo will be a big help so you should add that. it can be used to help all of their needs, it will help hold soil (you still need something else to hold it though), it can be used for building all kinds of things and naturally stores water. you need large patches of soil to grow it though.
If your locals are smart they might take an interest in their host and help feed, clean, and protect it, maybe even take care of its young.
You probably want them to trade or raid for metal and/or stone for tools, they might be able to use the pieces of the shell but they will need some stone tools to cut and shape it. the complete lack of stone means even shaping bone tools is tough. I can almost see them harvesting every stone they can find from uninhabited islands.
Do you mean, can a human build a symbiotic relationship with a supermassive tortoise? Certainly it can. The tortoise would have a huge diet, so it would probably be happier in a lush forrest full of habitation to graze all day. It helps fertilize the soil in it's path very well, creating areas that it travels back to annually. This is a system of regularity and habit that your people will rely on and have even mapped and given meaning in a similar fashion to the constellations (in reference to the technology required). This could be a kind of science innovated by your great geniuses. A trip over the ocean would be dangerous and require a lot of preparation. It wouldn't be done unless the tortoise's pattern declared it fit for the future, most likely for it to reach a mate. That's where the symbiosis comes in. The tortoise needs you to store enough nutrition to allow it to make it's journey across the sea. It would only be in this new location for a time due the regularity it must return to, the foliage it most enjoys and the habitat your people are most adapted too.
Same as any other island. It’s probably already covered in other colonists and a thin layer of topsoil. Just don’t dig deep, and you might not even notice it’s not a normal island.
You’d have a hard time preventing debris from building up on a large open surface, so that should not be surprising. Furthermore, some animals intentionally decorate their shells with local debris so it blends in. I don’t expect a tortoise to do any work, neither preening nor planting, because it can’t reach with its mouth and doesn’t have grasping hands.
Since you said “tortoise”, not sea-turtle, I expect it will be standing in shallows. It might move around slightly, but even normal tortoises are famous for moving slowly. It will be confined to the area that’s deep enough to releive its weight yet shallow enough to stand on and have its head above water. So, it’s probably some kind of reef ecology. It will probably be blended in with the part of the reef that’s above water, too.
Perhaps it lives in the shallow lagoon of an atoll. That gives it a nice pool to live in, and it is adjecent to land. And, south pacific atolls have a history of giant monsters caused by atomic bomb testing. Some of them are rather benign and even friendly, I suppose.