A Strange Question But I Have

Well, I'm not sure if this is the right place to ask, but here's my question:

First, I want to study animation and create a studio to make anime, but I want to learn a little more.

The latest chapter of Oshi no Ko came out, where the four bald guys who follow it were waiting for Kaguya's cameo, a cameo that ultimately didn't happen, which angered a good part of the Kaguya fandom.

I read a little about it, but I'd prefer someone who's an expert on the subject to answer this.

- I read that the copyright of a manga and an anime aren't the same, and that's why it's not very easy to do crossovers that occur in a manga within an anime. How true is that?

- There are exceptions, like Taiga's cameo in Eromanga Sensei (Toradora is from J.C. Staff and Eromanga from A-1 Pictures, but they're licensed by the same publisher in the case of their novels, so the dynamic is the same). How did they manage to do that cameo, which is the only one I can think of (besides the works of...) Key and Gintama)

What would have had to happen for that cameo to have occurred?

Regards