The item spawn is not as random as it seems at first.
There are nine zones and an equal amount of basic resources spawn every day at a random spot inside each zone.

To prevent the map to get polluted resources only spawn until day 10, after that the only way to get new resouces is by resource farming, searching in other zones or buying them from the merchant.
Of course there are limits to make sure some ares don't get cluttered with resources.
Every basic resource can only spawn up to a certain amount of times in an area, every resouce spawned in that area counts to these limits until they are picked up or removed in another way.