https://angularfirebase.com/lessons/build-an-angular-library-with-ngpackagr/

Create app

ng new my-app

Generate a library

ng generate library awesome

Build it

ng run awesome:build

Install it in your app

npm install dist/awesome

//if service needs external modules like HttpClient, create a module w/ that imported and inject into your client module that using the library

publish build to npm

npm publish dist/awesome

in another app, install npm package

npm install awesome