npx create-react-app executes create-react-app binary, and create-react-app uses yarn to create your project (if yarn is installed). that’s why you can see yarn.lock and also why yarn-start works.
Difference between
npx create-react-appandyarn create react-app
They both executes create-react-app binary. create-react-app is the one who decides whether it wants to create the project with yarn or not. to use npm in create-react-app, use --use-npm flag (no matter you execute create-react-app with npx or yarn or directly, you should set it if you want it to use npm .):
create-react-app my-project --use-npm