You can view message headers like this:
- In message view, Menu -> View -> Show headers
- In message list, under the "three dots" icon when you have exactly one message selected
In normal operation, AquaMail does not download all message headers. And even if it did, it wouldn't be putting them into message text (when forwarding), they're headers after all.
So please use "Show headers" as described above, there is a "Copy" button there.