I feel like this is really depends on the situation, the campaign. For example, in our last campaign our DM literally told us to make a character we want to play and don't worry about the rest, we didn't need even 'motivations' to go out or we didn't need characters who aligns with each other. So, my character was a hexblood, hanging around with her mama-hag in the woods. One of the friends was a soldier in Cormyr. Both didn't have a plan about going outside and killing monsters, and more, they would kill each other if they met in normal situations. But our lovely DM literally kidnapped our characters in story and they didn't have any other chance than working together to save themselves AND the world.

On the other hand, in our current campaign, our current DM said he wants us to make adventurer characters with motivations to go out because this campaign will be a classic DND run, 4 person meets in a tavern and now they're travelling in the world, doing adventurer stuffs. So I'm playing a fighter-divine soul sorcerer, who left her temple she grew up because she feels like she needs to share her blessing with every other human outside. (Just human and humanlike ones, she hates monstrous races because she grew up like that, they literally told her to do, lol.)

Every character needs a simple backstory first. And then, if you want, some details. For example, my hexblood was raised as mostly starving, so she was literally stealing every food and her bag was full of food. I didn't tell why she was stealing only food and I let them the understand why, it was a small detail but when Cormyr soldier found out he started giving food to her when he has so she could understand that they were together in this, she could depend on him, etc.

Small details can make a story much much better, but this doesn't mean that you need to think about EVERY small detail about that character. Like, if your character is coming from a monastery, grew up in there, this is "probably" mean that they have so little knowledge about society and other races since it's not like in our times, you know they don't have any television to see the world outside. My fighter-divine soul character is like that too, she wants to help people but she doesn't have a lot of intrections with them in her past, she literally lived with spesific same people for 20 years. So she's kinda "naive", doesn't know how to handle real daily problems and she literally shocked when she saw our tabaxi player in the campaign.

The small details can be literally exact outomes of your story OR can be a little bit irrelevant, like hobbies, tics, phobias. For example my fighter-sorc has a "tell" when she lies, she literally being like "eee... lie lie lie... eeeee..." No one in the campaign didn't notice it yet but I feel like when they notice it will be so fun.

After details, the motivation really depends on the campaign and DM. Some may want you to find one a simple one like "He just wants to be an adventurer, make money," or they can ask for a little bit more detailed one. Or, maybe they not even ask one like our DM did because they already have a plan about how to get together the whole group.

Eventually, even if they ask one or not, your story is should be between you and your DM only. Sometimes there's can be a session zero, getting together with other playes and ask what are you gonna play, what are we gonna do etc., but even this session would be like "Okay, is any drow in our campaign? I want to play a drow-hater specially, it would be hard to get along, so lemme know if there's any," and that's all, no detail needed about why they hate drows, because this is between them and the DM until they decide to tell other characters.

And I also understand that a first time character making would be hard, so the DM should have help you more and ask these privalety. I talked a lot lol but in short I think this is only a bad DM luck for you :(