It's not possible to "extend" the list resolved by one of the subgraphs, by pushing the items. The only "extend" action that is allowed is to update items of the list, not the list itself.

In GraphQL federation the general rule is that a shareable field is semantically equal, so requesting a field in subgraph A or subgraph B should result in the same response. Whether it is Apollo Federation spec or the one GraphQL Foundation (with its Composite Schema Working Group effort) is developing, the rule is the same.

It does not mean that the field is resolved only once, always. It could be requested by the gateway multiple times, each time the gateway could ask to resolve that field in many subgraphs, to resolve part of its inner structure that is only available in those subgraphs.