We (a FAANG) are actually looking for a dev experienced with Unreal on mobile devices. If anybody sees this and is interested, hit me up. Contact info in my profile.
Edit: Added emphasis that we're looking for an Unreal mobile dev, not Unity.
I used both Unreal and Unity at FAANG. VR and AR apps are obvious use cases but you find the engines in all kind of random places. Especially common in ML research groups for scene and environment understanding
Nintendo and Microsoft have their own custom engines and toolchains for AAA games. Unity gets used for casual titles where teams are smaller, roles are less specialized, and high performance rendering isn't as big of a priority. (Not that Unity is slow, it's just behind the curve of Unreal and the custom engines of big studios)
The safe bet for Breath of the Wild is to assume that Nintendo used an in-house engine. Although it is public information that the dev team used a modified version of Havok as the principle physics engine.