I agree with @SleepingPoppy
you can use irl people you know and borrow some of their ticks and quirks
but as everyone said above ^^^ background and goals helps a LOT
but I wouldn't agree on strictly using motivation/background
the best way I think you could make a character different from one another is also just, as silly and simple as it sounds. is to use completely different personality traits? well also a different variation of ideals.
like, here's an example:
scientist bob and scientist willy are both scientists, both of them are from rich families, both of them are in the search of knowledge and help make a better world, therefore they are scientists in that regard.
Both of them are working towards a goal to cure cancer, they both hate to see people suffer.
Bob, however, is more outgoing, and charming, whoever he talks to make that person feel like the most important and meaningful person in the world, which lines up with his character. he cares about people.
Willy, however, is the opposite, he is aloof, less talkative only using grunts and hand waves to talk to people, Willy wants to devote all his time and effort to curing cancer for the betterment of humanity, even though he comes off as a jerk, he has good intentions.
Bob wants to cure cancer by using the questionable act of human augmentation through bioengineering life, essentially warping the human form into something completely different, while Willy wants to devote his time into a more ethical method, but that would take years or even decades, and even with that, it's not guaranteed. while Bob's method is faster, has instant results, yet is ethically questionable, as these people aren't even the same person anymore. these characters have the same background and goal, but are completely different people.
now see what happens if both characters have been up to date with the other's works, and you put them in the same room, they meet for the first time, and seeing their methods, both of them have very strong opinions of one another.
iterating on that last point ^^^ doesn't matter if you have 8 vastly different characters, none of it matters if none of them bounces off of one another in any meaningful way, they are just there, instead of existing with each other, that's kind of the point of having vastly different characters, is that bounce. and even with that, there's nothing wrong with having less foil and making some characters similar, hell, it gives a reason for one character to relate to or even be friends with another character. as long as the personality isn't like the same (unless they're actually clones grown in a vat, or a hive mind) and it doesn't mesh so hard that they are essentially a copy paste of another character (in your world).
like, using an opposite example
a cop from a rich family, white knight, wants to enforce the law and safety of others, and a criminal that is an anti-hero, from a broke af family, is straight up an anarchist. one has a high and mighty personality, the other is more trivial, and shy. one is a white supremacist, the other is a social justice warrior, and lastly, one loves jewlery and is greedy, the other only want the essentials.
they both meet and realize their immense passion for star wars, like that passion is so freakishly immense, so much that they even have the same underwear of luke skywalker, now they don't wanna shoot each other, because apparently in this world, legitimate star war fan bois are rare these days. and they finally meet a person they can share their passion with.
boom, a bounce, two unlikely characters possibly become friends
and two characters who would on the surface seem like they'd be friends, end up being unlikely enemies.