how do you make a character feel like an actual person and not just a character that you're reading on a page? I'm aware that they should have likes/dislikes, flaws, a dialect, etc. But what else could I add to make them more relatable and feel like real people?
A real person has a sense of self-interest
In The Lord of the Rings, there are scenes where orcs get into fights with other orcs, or orcs and goblins get into fights. Why? In one case, because the orcs serving Sauron and the orcs serving Saruman have different goals, different masters, and will be punished or rewarded for different outcomes.
In a horror movie, when the person has to open the door to let the monster in, so everything goes wrong, the audience will often complain that they would never do that. And they probably wouldn't, because they're real people with a real sense of self-interest.
Even when a man sacrifices himself for another, it's generally because he knows the person, or wants to be kind. The action is still centered around motives that the character has.
A real person is (mostly) consistent
People are complex, with many motives which may compete with one another. ("I want to be faithful to my wife." "That other woman sure is hot.") It is usually your job as a writer to present your characters in a way that their reversals, when they come, make sense in context. In real life, we don't know all of a person's motives, but in a story we usually can be shown all the relevant factors, in advance of a decision.
That is, if the man in the horror movie opens the door to let the zombie of his beloved wife in, because he cannot cope with acknowledging she is dead... Well, it's consistent with his character. It's less contrived (probably), and therefore you are less likely to feel that the character is acting according to the needs of the plot, and not his own motives.
A real person has flaws
In our daydreams, we may know all the answers, always make the correct choice, be totally fearless, and never have someone say something that makes us look bad (even when we were actually right, dang it!). But a story is (usually) not supposed to be like a daydream, where everything works out and there is no resistance.
If a character gets impatient and loses her temper, or has to take some time off to just cry after finding out that her father was eaten by a zombie of her mother, or gets scared and runs from the orcs, she'll be far more convincing than if she is flawless and unassailable, always having the answer to every problem.
An author sometimes cheats
With all of the above being said, sometimes the author cheats, making sure that the character who's supposed to be cool doesn't get stuck in situations where any outcome is embarrassing. Sometimes the author has the character stop and feed a hungry dog (and not get bitten and get rabies). It's possible for you to get bogged down too much in realism, and forget that there are other techniques to make a character appealing, besides just realism. (And, occasionally, realism makes a character less sympathetic - because there are totally realistic, selfish reactions which make a character seem unappealing if honestly portrayed. Because yes, there are some scummy people in the real world, and everyone is at least sometimes less sympathetic.)
They need to have lived experience--a past and future, obvious to the reader. They need to contextualize events they experience into the broader setting. This means understanding the other characters in the scene, and imagining how it would feel to be in those shoes. They also need to be working toward a goal, and responding appropriately (emotionally and physically) to stimuli of various sorts. They need to share vivid specifics with the reader.
He hadn't felt so embarrassed since that day in third grade when he'd wet his pants at recess. Everyone had laughed, and it felt like that now, with the hot flush creeping up his neck. He said to his boss, "I didn't realize the customer had to sign all three forms. It won't happen again."
"No, it sure as hell won't."
The man was angry, but Jensen understood. He'd cost the company twenty five hundred dollars with his stupid, boneheaded mistake. "Please, sir, give me another chance. I have to prove I can hold a job. If I can't..." Jensen couldn't finish the sentence. If he lost this job, he'd be out on the streets.
The boss was still glaring, but after a moment his expression softened. He looked away, seemed uncomfortable, and after another moment said, "One more chance."