It says that header region is included as Region on the Page, with works.
header.html uses "Include page (Header)" Page template & "Include page (header)" Page Schema.
That Page schema contains only 1 region, Header:
and Region Schema "[Header] Region" contains Component based on Schema "Menu".
Component is filled in region "Header" on the header.html Page. note: Core example has here 3 calls to the nested regions.
Why need to call here @Html.DxaRegion("Header") again to get it work?
Region view header first time contains model see figure (Regions count 1, Entities count 0) When execute
@Html.DxaRegions() same view is called.
Second time @Html.DxaEntities works (it is levelt up).