To be clear, I don't have a smart phone so I wanted to know if it was possible to do development without one. Unity sounds like a good solution. Also, I don't understand why you'd have to build the game to run it in an emulator but you don't if you want to run it on a phone. I think I'm not clear on the difference between compiling and building.
Compiling and building are the same thing. They are the time-consuming part. Unity has it's own "native" mode in which it runs the game you are building, and that doesn't require a build to test it, so you can quickly iterate inside Unity, but to build e.g. a standalone windows EXE or Android APK, that takes at least a few minutes per time, so you only do this every now and again to detect any compatibility issues that Unity missed.
Think of it as the difference between working in a video editor suite, vs having to export the final movie. You can work in realtime inside the editor, but the process of "mastering" the video is going to be slow. It's similar with making games in Unity vs exporting them as completed games. You
can completely build a game in Unity with the intention of it being a mobile game without ever exporting it, but you will need to actually put it on a phone one day to test that it
actually works as intended. After all, Unity doesn't have the same RAM limitations as a real phone, etc.
Emulators are possible, but you need a beast PC to run one effectively. But they'll never fully replace an actual phone, because testing the touch controls still needs are real phone. An emulator however will let you run the game with various Android versions, CPU and RAM settings so they can give you a better idea of minimum spec without needing multiple phones.
But generally you can break this into phases. (1) build you game for Windows inside Unity, export occasional windows builds and test those (2) tweak the controls / scaling so it would look good on a phone-sized thing (3) stress-test it in an emulator, (4) give the apk to some friends with a phone, test how controls feel.