The players dont need to. Everyone knows what an Elf or Dwarf or Hobbit Halfling are.
Do they?
I started playing D&D in the 90's and even in that paleothilic era, my intro to fantasy was Final Fantasy, not Lord of the Rings. I knew more about moogles than hobbits. And when I did start playing D&D, it was through BECMI where there was exactly one type of elf.
A player today might know what an "elf" is in the general concept of fantasy, but I suspect they don't know what a sun elf, or a areneal elf, or a qualensti elf are and what the difference between them are. And the game doesn't do anything to help them. Look at the first paragraphs from 2024:
Created by the god Corellon, the first elves could change their forms at will. They lost this ability when Corellon cursed them for plotting with the deity Lolth, who tried and failed to usurp Corellon's dominion. When Lolth was cast into the Abyss, most elves renounced her and earned Corellon's forgiveness, but that which Corellon had taken from them was lost forever.
No longer able to shape-shift at will, the elves retreated to the Feywild, where their sorrow was deepened by that plane's influence. Over time, curiosity led many of them to explore other planes of existence, including worlds in the Material Plane.
A simple and succent little origin story. That is contradicted by Eberron (where elves were slaves of the giants), Krynn (where elves were made by a different god) and Athas. Not to mention Ravnica (which uses MTGs origin for elves). And I guess its true for Ravenloft, Planescape and Spelljammer (due to their multiverse elements) but so are all the other origins. And when D&D has attempted to lay down common lore (like the First World) setting purists balk how Bahamut cannot be the same on Oerth, Faerun, and Exandria, let alone be Paladine too (while also being a constellation in Eberron).
And as steeped as I am in D&D lore, I still needed a wiki to type all that out. I can't imagine most players would even give a flying fig to get that deep in the weeds!
So yeah, Bob the new D&D player might know what an "elf" is, but his version of elf is probably not going to align with what the version of elf your campaign is running. And that mis-match of expectation is where these "Bob created a huge backstory that doesn't take into consideration my world" problems come from. Well no kidding, D&D barely gives him anything to base it on and then proceeds to produce a half-dozen exceptions to the scant lore there is!