https://learn.microsoft.com/ja-jp/dotnet/api/system.xml.linq.extensions.descendantnodes?view=netframework-4.8.1&viewFallbackFrom=netcore-2.2
Extensions.DescendantNodesT(IEnumerableT) Method (System.Xml.Linq) | Microsoft Learn
Returns a collection of the descendant nodes of every document and element in the source collection.
extensionsmethodsystemxmllinq