The origin(s) of orcs were explained two different ways (i.e., inconsistently) by Tolkien: the orcs were either East Elves (Avari) enslaved, tortured, and bred by Morgoth (as Melkor became known), or, “perhaps… Avari [(a race of elves)]… [turned] evil and savage in the wild”, both according to The Silmarillion.

The orcs “multiplied” like Elves and Men, meaning that they reproduced sexually. Tolkien stated in a letter dated 21 October 1963 to a Mrs. Munsby that “there must have been orc-women”. In The Fall of Gondolin Morgoth made them of slime by sorcery, “bred from the heats and slimes of the earth”. Or, they were “beasts of humanized shape”, possibly, Tolkien wrote, Elves mated with beasts, and later Men. Or again, Tolkien noted, they could have been fallen Maiar, perhaps a kind called Boldog, like lesser Balrogs; or corrupted Men.

https://en.m.wikipedia.org/wiki/Orc

Huh, interesting. I knew about the Morgoth corrupting them part and also making them of the earth and/or beasts, but somehow I’d forgotten any descriptions where Tolkien writes that they breed like normal Elves and Men.