Indesign 20.5, Windows 11
Problem: it's possible to drag & drop a master page from one opened document to another opened document, but only if the documents are arranged side by side. For comparison: in Photoshop, if you want to copy a layer between documents, you can simply grab it, move the mouse pointer to the other document's tab, which would then switch to that document and then you drop. Not so with Indesign. In the same way of thinking it should be possible to drag & drop ID layers between documents, but this doesn't work at all.
Please align such behavior between your tools.