i had this issue with my 4 year old cat my roommates 1 year old cat. we just came to conclusion that my cat doesn’t like other cats when they bother her cause they were only chill when they were doing their own thing. some cats, also probably like mine, just growl more than others and you just have to read the body language for when to separate them
there is also the thing to get them used to each other’s smells by feeding them between a closed door and work your way up to being able to feed them in the same room. you can also use favorite toys as a way to distract them from each other, especially if one of the cats is very energetic. but it’s always a good idea to have things that they know is their own, like cat beds/trees that already smell like them in shared spaces