While all of Nintendo's official emulators are afaik made internally, it's interesting to note that Sony used an existing open source emulator (PCSX ReARMed) for their Playstation Classic.
> While all of Nintendo's official emulators are afaik made internally
I believe the consensus is that the NES classics on GBA were done using an open source emulator. The emulator itself was liberally licensed and I think someone asked the author's thoughts and they were 'ya, that's half the purpose of the license. i don't care'
https://arstechnica.com/gaming/2018/11/sony-using-open-sourc...