Cats and dogs are very social animals, but often people don't realize what an individual thing that is. I have four cats, and each cat's relationship with the other three is different, because they all have different personalities and different roles within the clowder. They get along fine, but if you observe them, you can see the dynamic between each pairing and the relative status of each cat. I learned long ago that three cats stuck in their ways have a hard time accepting a new adult cat moving in with them, but they always accept a kitten, with the youngest of the three becoming a substitute parent. This has been going on in my home for thirty years, so I believe they learn this behavior from each other.
The dynamic is different with the family in this video. Ollie came to them as a kitten with special needs. The other cats, Bosco and Peanut, did not like Ollie. But the Pomeranian Max was willing to be Ollie's friend, and soon best friend. We don't know exactly what is going on in these cat's heads, but we are glad that Max opened his heart to Ollie, which over time, opened the door to other relationships within the group of pets. You can keep up with Peanut, Bosco, and Ollie at their Instagram page.