Most-Grown Fruits Worldwide

Oranges are a hybrid of mandarin oranges and pomelos, as are grapefruit, but with more pomelo contribution.

Lemons are a more complex 3 way hybrid between all three, as are the more exotic citrus like bergamot.

To caveat the above, some varieties of mandarin orange sold in stores do have some degree of pomelo admixture, but regardless mandarin oranges are not a crossbreed of oranges. The inverse is closer to the truth.

Here's a neat chart that visualizes some of this: https://en.wikipedia.org/wiki/Citrus_taxonomy#/media/File:Citrus_tern_cb_simplified_1.svg

At least clementines and tangerines are closely related genetically and visually. Clementines just have a bit more pomelo DNA.

They also grouped pomelo and grapefruit when grapefruit are pomelo hybrids with substantial mandarin orange DNA, and are closer genetically to oranges than pomelos.

Lemons and limes are also distinct hybrids.

But citrus DNA is so messy and overlapping that some degree of more-or-less arbitrary grouping is necessary unless you want to either have a single category for all non-kumquat citrus, or treat every cultivar as a distinct fruit.

This quiz seems to divide based mostly on appearance and taste rather than genetics, which is fair IMO.

Generally speaking, fruit in the culinary sense means the sweet and/or sour-tasting produce of certain plants. Nuts are hard, oily, non-sweet produce that grow in shells; vegetables are other savory or non-sweet plant produce; herbs and spices are plants parts primarily used for the flavor they impart (as opposed to their actual matter); and grains and cereals are small, hard, dry seeds from grasses. The edges of these categories can be fuzzy — carrots and sweet potatoes are both sweet, but are treated as vegetables, for instance — but it's generally how things are classified. Tomatoes, cucumbers, bell peppers, etc. are primarily used for their savoriness, and so are, culinarily speaking, not considered fruits.