After npm start if you want to run your project first you have to eject your project for that you have to run the command "npm run eject" in your command prompt
It will give you 3 options as shown in the image.
If you want to go with 1st options it will run your project in the Android emulator by asking u some basic questions like your project name and other stuff.
If you go with the 2nd option it will run the project in emulator but with the expo.
try this solution hope this helps.
if you're trying to run the app in an android emulator (without expo) then you have to use the command
"npm run android" to run your application.