I stand corrected. The 2015 and 2016 does seem to have some software differences(affects the 13.5 update) so I will only say that 2015 models are Windows based for sure.
Note: The head-unit from a 2018 model has it own firmware, so yes it should work if the unit can fit into the 2015/2016 model and have the same connectors. However, who will spend the $$$$ to swap the older stock amp to the newer stock amp located in the 2017+ models just for
Carplay since the older Lexicon amp does not work with the newer 2017+ head-unit.
My point is that it is highly unlikely the 2015/2016 models will get
Carplay/
Android Auto from an official Hyundai update. Maybe 2016 model, but not the 2015 without a doubt.
However, if some used 2017+ head units and Lexicon amps can be found then yes it seems that both the 2015 and 2016 models can be physically upgraded without breaking the bank. As it stands the 2017+ Lexican amps are about $1000+ new. A lot of money just to gain
Carplay/
Android Auto in my opinion.