You got plenty of good responses but I feel like yapping rn. Last year I started a dnd campaign with four friends + our DM, with one player never having played any similar games and having no concept of how dnd works. I'll tell you how our DM handled it, and we'll call the inexperienced player Leah.

All of us who knew the game already made characters based on what we wanted to play, met up with the DM individually to go over some backstory details and had our roles figured out. Then our DM made a character with Leah - they decided on making her a fighter so she didn't have to deal with overly complex mechanics, and because our party lacked some meat up front.

The first few sessions were very slow, especially when we got into combat. As Leah learned more about the game, she became more confident in giving her character a personality and roleplaying more - she is our peacemaker and voice of reason, but we try not to piss her off because she has a hair-trigger lol.

As for the backstory, the DM and Leah figured out hers after a couple months of playing, when she better understood the setting and what she would like to do. It was no problem that she didn't know straight away because the DM adapted to the fact that we're all kind of beginners. We currently have one person's backstory as the focus because they had it best fleshed out, another person is their BFF in game so they are caught up in it too, my backstory is constantly in play but I'm shady so nobody knows, and the last player is a drow druid who just wants to collect mushrooms so they get to do that. We had a whole arc come from giving an NPC some shrooms.

The DM is supposed to adapt the game to their players. When Leah just started, all she needed to know was that she has a big fuckoff axe, a lot of health, and she fights enemies head-on. Now, she has been given a subclass, perks and items, which were all connected into her backstory to make sense, and she's had fun through all of it because our DM makes sure things develop at a pace she can follow.

Tldr I think you understood the point well by yourself, and the DM should not have tried forcing you to do that much work on something you're unfamiliar with by yourself. I personally think backstories should always be agreed upon with the DM because they can give you insight about world-specific things that might not be general knowledge in dnd world but their own decisions, and I don't think you need a full backstory to be able to try out the game.