Here's the quick version.
All large fruit trees needs to be pollinated to bear fruit. This means an insect (often bees), need to visit a different tree and transfer pollen between plants. Only pollen from the same species (ie: apples, plums, pears, haskaps etc) is compatible, and it also needs to be from a different variety (ie 2 Gemini or Goodland apples won't cross pollinate).
The reason it needs to be a different variety is that all trees are made from cuttings and grafts of each other (essentially meaning they are all clones). So planting two Gemini apples next too each other is no different than 2 branches on the same tree. For things like apples and pears all varieties have overlapping bloom times and any two varieties are compatible. Apples also includes crap apples which are great pollen sources.
Plums and haskaps need to have overlapping bloom times to ensure compatibility and there are many charts which I will post below that can be used to determine that.
Other plants namely- blueberries are semi-self fertile and will set fruit, but you can get up to 40% more with cross pollination.
There is a third type of cross pollination requirement being plants that require pollen from a male plant to pollinate flowers on the female which with produce fruit. This applies to us in Kiwi's and SeaBuckthorns. But also for many other larger trees, and occasional only males are planted to eliminate fruit set, but leads to more pollen and allergies, but that's another topic.
When it comes to distances, usually it's said about 400ft for large trees and 50ft for small shrubs. Also usually 4:1 ratios are recommended, so at least 1 different variety for every 4 planted. The ratios and distances can be significantly increased if you are a beekeeper or have a great deal of bees and pollinating insects around. Often other flowers are planted as companions to increase insects in orchards.
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