Artboards for different devices/orientations

I'd just like to open by saying I'm a beginner at this which is why this may seem a strange question, but bear with me. 

All the tutorials I've seen for XD create designs in mobile, portrait, one-sized target but I haven't seen any that reflect the design into Landscape or other device/viewport sizes.  It makes sense to me to do this but is that common practice?  Am I missing a useful insight?

If I am designing an app for iPhone and iPad, I would layout relevant artboards at two sizes to represent phones and iPads as well as both orientations.  For simplicity, the app would be designed to look the same on phones of all variants (iPhone 6 upwards) with altered design for larger viewports on iPads of all variants.

How do you handle the multi-tasking viewports for iPad: where it works, I'd stick with the iPhone size but if the dimension is smaller would you layout an artboard for that as well?

Final question: I've created artboards for portrait and landscape.  I created the portrait board first: icons I turned into symbols and just dragged them onto the landscape board, but other elements (input fields, buttons etc) I just copied and pasted.  Would it be better practice in this scenario to convert all common elements to symbols?  I'd assume that I could change them in one place and reflect that across all usages.

Thanks.