I had my set up in account A:
I moved my domain name to account B.
The hosted zone is still in account A.
Why is the traffic routed correctly - I can see the expected content when I access my website, shouldn't make sense to have the hosted zone in the same account as my domain?
Where can I find the docs to clear my mind, please.
There are two items to understand in your question:
The registrar for your domain contains the IP addresses or hostnames of the authoritative name servers. Those name servers can be located anywhere. Authoritative names servers are the DNS servers used to resolve DNS queries for your domain.
A Hosted Zone is a collection of DNS resource records similar to a database.
In your case, your domain is located in account B, with either hostnames or IP addresses that resolve to resources located in account A. There is no problem with that design and is in fact commonly deployed for security and isolation.