This is from Animal Farm
... and surveyed with speechless admiration the ploughland, the hayfield, the orchard, the pool, the spinney. It was as though they had never seen these things before, and even now they could hardly believe that it was all their own.
'It' is the farm. Although it contains the ploughland, the hayfield, the orchard, the pool, and the spinney, it is still one farm. It is considered as a whole.
You could replace 'it was' with 'they were', but you would lose Orwell/Blair's emphasis on the farm as a whole. He was a careful writer, and wrote deliberately. Orwell uses a farm to make an analogy with the Soviet state. In 1917 the Bolshevik revolution was deemed to have given the people (= the animals) control of the whole state (= the farm), as specifically one entity. It would be confusing and unfocused to say the people had control of Russia's rivers, mountains, forests, lakes, etc.
@Michael’s answer is excellent, but one more point to note: the sentence is
It was as though they had never seen these things before, and even now they could hardly believe that it was all their own.
The sentence already uses they and their, referring to the animals. If it was replaced with they for the parts of the farm, then the sentence would be using they for two different things alternately, making it rather difficult to read. Keeping the animals as they and the farm as it gives a much clearer and stronger sentence.
In this case "it" and "all" work together to indicate that all the previously listed things, taken together, are the "it" that we're talking about.
The phrase "it was all their own" could be rewritten (somewhat less poetically) as "all of it was their own", which might make it more clear that we're talking about a singular "all" rather than a collection of individual bits.
