Citron
A source of confusion is that "citron" or similar words in French (and other languages), and English are false friends, as they refer to the lemon. Indeed, into the 16th century, the English name "citron" included the lemon and perhaps the lime as well.
Citrus
The large citrus fruit of today evolved originally from small, edible berries over millions of years. Citrus plants diverged from a common ancestor about 15 million years ago, which was about when it diverged from the closely related severinia, for example the Chinese box orange. About 7 million years ago, citrus plants diverged into two groups, the main citrus genus and the ancestors of the trifoliate orange (poncirus), which is closely enough related that it can still be hybridized with all other citrus. These estimates are made using genetic mapping of plant chloroplasts. A DNA study published in "Nature" in 2018 concludes that citrus trees originated in the foothills of the Himalayas, in the area of Assam (India), western Yunnan (China), and northern Myanmar.