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Sure, I understand them, but that's because I've been thinking about them.

But how do I transfer that knowledge to the reader without taking the time and space to fleshing them out further??

The standard answer is "stereotypes", because stereotypes are broad generalizations about humans, and without the ability to generalize we must start tabula rasa in every new situation, no matter how similar it is to situations that we are already familiar with. (More importantly, stereotypes have a basis in fact -- or at least reporting -- no matter how partial, one-sided and/or out-dated they are.

But since stereotypes have been declared Evil, I need so other method of KT (Knowledge Transfer) about these characters.

Thanks

There is a difference between stereotypes and simply not fleshing a character out.

For example, your main character goes into a coffeeshop, orders, and sits down to read. At the next table are two women he works with. What are they talking about?

Mind you, you're not going to get into their heads. The narrator won't give background about them or tell their point of view. They may or may not speak to the MC (if they do, it is probably quite brief). The reader doesn't know their hopes and dreams, and doesn't care.

Some gender-based stereotypes could be that the women have secretarial/clerical positions, aren't really into the work, and their favorite conversation is gossip about other women they know or their husbands/boyfriends.

Some women (and some men!) have one or more of these characteristics, and that's okay. It's a stereotype when most of the women in your story fall into this or another trope, with or without some "girls who are not like the other girls." These aren't the only gender-based stereotypes. The women could be young, scheming professionals, who are gorgeous and fashionable. Just to name one of many possibilities.

Take a minute to think of them. In real life, what sort of women might sit together at a coffeehouse during lunch (or before or after work)? Of course, the possibilities here are endless. Choose any one of them. Maybe one is the owner of the company who is grooming her daughter to take over when she retires and they're talking about problems with a client. Perhaps they're analysts with cubicles next to each other and they're talking about taking their kids together to the upcoming county fair. Or they could work in the company cafeteria and go to the coffeehouse so they don't have to make their own coffee, thank you very much, and one is inviting the other to watch her race motorcycles that weekend.

It doesn't really matter what you choose because they're not important characters. They're background, like the coffeehouse. I didn't describe the place but you already know it's the type of coffeehouse with chairs and tables where people can spend time chatting or reading. Say I make a comment about the MC being glad that the pie of the day was peach, so he got a slice with his iced mocha. Now you know the place is local, not a chain, caters to the modern whim of different coffee choices (so not an old-fashioned diner), and, yes, it's probably late summer.

With characters it's the same thing. Choose a couple details and suddenly the reader has an image in her/his mind. (Whether it's the same image that's in your mind is not important.)

A way to do this with stereotypes is to make all the non-primary characters fit neatly into tropes and other expectations, then make the primary characters ones that don't fit the mold.

The better way is to see non-primary characters as the same diverse individuals people in real life are and throw in details to match. You don't need much for background characters, like the women at the next table. Secondary and tertiary characters will have names, descriptions, and roles in the MC's life. But they have their own lives and they don't just exist to further the MC. Write that in.

You don't need much visible fleshing out, IMO. It's all in the details.

Take the 'wild party friend', for an example, who swoops in and drags the MC into a crazy college party where everything goes.

At a certain moment, the MC tells the friend, already getting a bit beyond tipsy, they shouldn't be there. There's an exam coming up and... "shouldn't you worry about that, too?"

But the friend shruggs.

"Why bother? You have to relax and enjoy the fun while you can. You know, my neighbour keeled over because of a bad heart at 20. He didn't even know he had a bad heart. You need to carp the day, or whatever it is they say. Do you know what your problem is? You need another drink. Come on, I'll get you one."

That friend may never show up again, but that one dialogue line broke him out of the stereotype mold. At least a little.

But maybe you have a shopaholic as a tertiary character who bails out on the arranged shopping spree because her niece broke a leg and she was going to spend Saturday playing at tea parties and hairdressers to make her feel better. But hey, can they reschedule the shopping spree to Sunday? She doesn't want to miss the sales and shopping alone is no fun.

Sorry, the MC can't make it Sunday, the friends text back. Maybe next week?

Thanks for nothing, girls, seriously.

Or perhaps there's an important meeting where the MC is going to make a presentation, and as the other participants come in someone mentions that Jones won't be coming because his son is in hospital again. There's no need to give explanations.

Just drop a line that shows those tertiary characters have a life with priorities that are as important for them as MC's priorities are important for the story. They can start as a stereoype or not, but give the reader a glimpse of their own stakes.

One of the most interesting stories I read has a 'Romeo and Juliet' kind of story happening in the background. The MC sees those apparent 'extras', hears some unimportant gossip about them... and in the end learns they were involved in a love story that ended tragically. Can't recall the author or title, though, as I read it decades ago. Those two 'extras' had far more important stakes in their lives than the MC! And even if we never really met them, we ended up feeling for their tragedy.