@Bluerose2020 i share your frustration. I think the reality is that all schools operate in a slightly different way, which is part of the reason it seems so opaque. The way that Emanuel assess kids seems slightly different from other schools - for example they interview a higher proportion of candidates. It would certainly be so much more straightforward (and easier on the kids) if there was a common assessment that was used for all schools.
I also think the relationships between junior and senior schools differ depending on the schools. Our primary headmaster seems to have stronger relationships with some schools than others. Ultimately I don’t think it matters how strong or weak communication/relationship between the schools is. The secondary school will want to take the most talented and capable kids regardless of what primary they are in.