The forbidden fruit was a special unknown fruit that grew on the Tree Of Knowledge Of Good And Evil. The tree was there for a test; and if Adam & Eve has passed the test, the tree would have been removed (and we wouldn't be in this mess right now).

If it had been an apple tree, the Bible would have said "apple tree" and called the fruit an "apple". God's people are the apple of His eye, so something that plunged His people into sin was not an apple. We still have apple trees.

We won't know until Jesus creates the New Earth and we are treated to the whole panorama of earth's history, beginning with Creation week of Genesis 1. Everything now is speculation and the theories people come up with lead to misconceptions about dozens of other things.