What if players don't know the setting very well. How do they write a backstory that fits? How do they write a backstory that's relevant?
*Well, talking to the DM &/or other players helps.
We spend considerable time BSing during our games. It's not "Sit Down, Roll Initiative, OK it's 10pm here's some XP & thanks for coming." Since we're already discussing random stuff we might as well discuss character details too.
*You can Google just about anything! While sitting at the table waiting for the turn to cycle back to you!
*But nothing beats the time honored method of Make S(tuff) Up.
I mean, when we started as kids with the Basic set all those years ago? How do you think we came up with background details? About
anything? That's right,
we made it up. Often inspired by something we knew about (or thought we knew about

)
As a DM? When a player doesn't know a setting too well? I just have them make up something & generally take care of making it fit on my end. If it's something too far afield we'll discus it a bit & something will be worked out. It'll be OK. And as they play? Then, like the rest of us, they'll acquire more setting knowledge & their next character will have a better(?) background.
Almost every game I've ever played in has had the players backgrounds created on the spot during character generation, and sometimes they get further detailed in play. We talk about what the setting is and about what type of characters we want and then we make them to fit.
I would be a little shocked if a player handed me a written backstory. What am I supposed to do with it?
You're supposed to mine it for ideas of course! Use details to help bring the world alive. Maybe something will spark an idea for a story arc. And discuss with your player if there's particular bits they want featured in the game.
Ex; In my Thur night game I've got a 1/2elf minor noble* from someplace called The Octopus Isles*. She's a socialite/party girl who ran away from court* to attend a Mardi Gras/Carnival type event*
. She got stoned*, made a pact with some entity that she perceives as looking like a glitter covered salmon*, was then transformed into a purple winged Tiefling by said salmon*, and became a 1st lv Warlock*....
She then joined up with several adventurers she met** and headed off into the Southern Woods to "Save the Forest Queen"***.
Humorous adventures have ensued since.
*supplied by the Warlock player
***supplied by another player
**supplied by me, the DM: "The game will start with you in attendance at a Mardi Gras/Cranival type event where you will meet the rest of the what'll form the adventuring party. (and, knowing the players I've got) Try & keep this PG13...."
So. Now with this character my world has had an Island chain, a specific Noble Court, and a salmon shaped god/patron added to it. OK.... What to do?
1) Detail the Octopus Isles & it's Noble Court. (work with player on this)
2) Play the part of the inscrutable entity known only as
The Salmon God.
3) Soon the party will discover that the Warlocks family has dispatched a variety of bounty hunters ala Empire Strikes Back to bring their daughter home.
And I've got stuff from 4-5 other players to work into play as well.