So, it kinda works like this: If an artist believes that somebody else has infringed on their copyright by say, tracing their work, or including a character who looks very much like a ripoff of one of theirs, they need to point out the similarities for legal people to judge.
The similarities cannot be genre tropes like "it's drawn in a manga style", or "this character is blonde and has a sword. My character is blonde and has a sword"; you have to be able to point to specific details that are distinct to your work and point to those details on the other person's alleged copy. These details are called "Unique Identifiers".
So because unique identifiers must be specific, it's possible to make something very close to another IP without getting sued. So say you want to rip off Fullmetal Alchemist. Arakawa cannot sue you over using the concept of alchemists; they're a real thing that existed in medieval times, but she might be able to sue you for the distinctive "making a circle with your arms by clapping and then touching the thing to magically alter it" way of doing "alchemy" characters in her series use. She can't copyright magic circles, but she can copyright her specific designs of them. She can't copyright the concept of a hero who is a short guy, a blonde guy, a guy with two prosthetic limbs, or a guy with a red coat.... But if you make the main character all of those things, so he's a short, blonde guy with a prosthetic arm and leg who wears a red coat... you might be in territory where she can, because those things in combination can become a case a person can argue.
But if my hypothetical ripoff story "Very Heavy Alchemy" features a protagonist who is tall, blonde and wears a short black jacket and is in a wheelchair, and he does alchemy by throwing bottles of potions he's mixed at things, that's legally distinct. It's a series with similarities, but they're not enough to get me sued.
With poses and things, you have to be able to point out direct similarities, and sometimes people just let it slide because taking this stuff to court is such sink for time and money, and if the person who copied you has more time and money, they can stall the case for years so you run out of resources. Plus if they can spin the publicity right and whip up a large or dedicated fanbase into a frenzy, it could be a PR disaster. It often becomes a big PR battle, where one side is like "This person stole my livelihood! Me! An independent artist! Stand with me to protect creators of original work!" and the other side is similarly going to the press or social media like, "This person is trying to clamp down on my artistic expression, and that they can copyright something as generic as a pose! Ridiculous! Stand with me to protect artistic freedom!" that might ultimately come down to who the press and public sympathise with more, because you could win the suit, but still destroy your career if the legal action makes you look bad and dominates all search results abut you.... Or it could be a massive publicity boost to your career and make you look sympathetic. Basically it's all a risky business and not to be entered lightly.
