Answers for "how to let npx create-react-app use yarn not npm"

create react app with npm not yarn

npx create-react-app my-app --use-npm

How to create react app with yarn, npx or npm

npx create-react-app app-name //With npx, the CLI uses npm to install dependencies
npm init react-app app-name // With npm, the CLI also uses npm to install dependencies

yarn create react-app app-name // With yarn, the CLI uses yarn to install dependencies