How to unload data from one index, excluding data from another index?

Question

PHP

There is an old index with 5,500,000 entries.
There is a new index, in which there are 5,800,000 entries, which, it seems, contain 5,500,000 old + 300,000 new ones.
I need exactly these new 300,000 (I need to move them to a third place that is not related to elastic) and so far the only sign by which I can calculate them (which I see) is to somehow exclude the smaller one from the larger index and display the difference ... Can this be done somehow? And if so, what should be the approximate syntax (or what chapter in the docs?).
Specifying numbers in such a quantity or names in must_not, as you understand, is impossible (well, or I don’t know how to do it).