TL;DR The fruit of the tree of knowledge being a literal apple is non-canon, being entirely based on a pun.

The word "apple" is not used in the Bible, that is, unless the Bible in question is a translation that specifically uses that word. Even then, see below.

The whole apple thing comes from:

1. the fact that the word for "apple" can be used as a synonym for "(any) fruit" in some languages and context, and so could mean any fruit.

Think about French pomme de terre for "potato" which is literally "apple (meaning 'fruit') of the earth". Dutch has aardappel (earth apple) which is the same thing. Fun fact: Old English eorþæppel (earth apple) allegedly meant "cucumber". Go figure. But I digress.

1. Latin is the main ecclesiastical language for one particularly influential branch of Christianity and one word for apple in Latin is "malus". That sounds like a lot of unrelated Latin words that start "mal-" that mean bad or evil, thus an apparent connection to the fruit of the tree of knowledge also leading to evil.

(I mean, it might actually be a proto-apple of some sort (modern apples did not exist 7000 years ago or whenever it was supposed to be) but the Bible doesn't specify.

Some scholars think that the whole thing developed out of metaphor for abandoning a hunter-gatherer lifestyle for farming. Others think that it might be a reference to beer / alcohol, which is one of the first things humans got interested in after farming.)