Assalamualaikum, I actually had this question, did the entire world submerged during Nooh's time, Nooh went on a mountain with some people and animals to protect from that flood,but did the entire world submerged during that or just some part of it? And if the world submerged completely, how was it restored?
In Surah Hud, we learn that the Ark of Nuh (AS) traveled on the water for many days until Allah said to the clouds to hold the rain and the earth to swallow the water.
The Qur'an 11:44,
And it was said, "O earth, swallow your water, and O sky, withhold [your rain]." And the water subsided, and the matter was accomplished, and it [i.e., the ship] came to rest on the [mountain of] Jūdiyy. And it was said, "Away with the wrongdoing people."
As for if it did affect the entire world or not, I myself am not sure about that. But I am sure that wherever there were humans the flood did affect, as Allah simply wiped out ALL the disbelievers and CARRIED ALL the believers in the Ark of Nuh (AS).