The plateau of Gorgoroth (the northwestern corner, where Mount Doom and Barad-dur were located) became an uninhabitable wasteland. The larger south and eastern portion, called Nurn, was a more fertile region where Sauron kept his slaves (who produced food for his armies). After the War of the Ring, Aragorn freed the slaves and gave them Nurn as their own land to settle.
1 - Yes, Aragorn freed all of the human slaves of the Nurn-region of southern Mordor - its more fertile there because all of the volcanic ash is great for soil nutrients, so they've got dry-area farming with irrigation and so far. Tolkien did answer the question of how, realistically, Sauron was able to supply and feed Mordor's vast armies; big slave-tilled fields in Nurn (the area around Lake Nurnen....though, just their luck, Lake Nurnen is brackish and salty, not a freshwater lake).
2 - the plateau of Gorgoroth, northwestern Mordor, is the heart of Sauron's domain; that's where all of the "heavy industry" is of weapons forges, metal foundries, mines, garrisons, etc. It was devastated by earthquakes in general, and in particular by massive volcanic eruptions from Mount Doom. Also, all of the massive structures which had been aided in construction by the One Ring instantly collapsed when it ceased to exist; i.e. Barad-dur and the Black Gate. Its still physically "there" -- you can't destroy "Mordor" the geographical location, you destroy "all of the castles IN Mordor" etc.
3 - As for the Orcs, Tolkien touched on that a bit in the original epilogue he abandoned, in which Sam is reading the story to his children years later (a draft of which is included in the behind-the-scenes "Histories of Middle-earth" book series volume "Sauron Defeated".
As he said in the book chapter when the Ring in destroyed, most of the Orcs died; they lost a lot of their guiding will when Sauron was defeated -- they're normally not all that smart or coordinated, think like how they were in The Hobbit.
Many turned their weapons on themselves, or on each other, and many were killed by the Free Folk as they ran around aimlessly.
Within a few years, there really weren't many Orcs left.
In the abandoned epilogue, Sam is directly asked about this by one of his children, and his answer is that they aren't extinct, there are still a few "if you go looking for them", but its really that the handful who survived are hiding in the mountains and caves, just a few tribes here or there, just living like brigands and highway thieves, but little more than a nuissance; again, think back to what they were like in "The Hobbit" attacking travellers in the mountain passes, but never enough to raise an actual "army" of any kind.
****King Aragorn (well, he takes the regnal name "King Elessar") leads Gondor into a new golden age, and uses its resources to rebuild its twin kingdom Arnor to the north (which includes the Shire) creating the "Reunited Kingdom". They're closely allied with Rohan led by King Eomer.
***basically it shifts from wars between Elves and Sauron, to wars between Men who allied with Elves and Men who had allied with Sauron.
Its vaguely mentioned that Gondor (the RUK ) and Rohan would continue to have (victorious) wars expanding against the Easterlings and Southrons who had allied with Sauron -- though they stress that "the Easterlings" or "The Haradrim" are groups of nations, not just one faction -- Aragorn sends prisoners they captured in the final battles back home to tell their leaders they want peace if possible and many only served Sauron out of fear and because he had conquered and conscripted them. There were still a few bad eggs though, so they had some wars against the worse ones.