For better DX strongly recommended to use npm@>=7.
It can be installed via npm i -g npm@7 and also it's built-in in node >=15 package.
Use npm run dev for development. (port is 3000 by default).
For development use the network mainnet, like this http://localhost:3000/mainnet
See packages/*/package.json script section for build, test, lint, etc.