How do Node Modules get added when deploying after pulling project from GitHub

I have never deployed before and am trying to figure something out.

I have a project that has NPM dependencies. Of course, when I push to Github, the Node Modules are not included. I have seen some videos on YouTube where someone uses Forge and pulls their GIthub project which doesn't include their Node Modules. Forge automatically runs composer install and runs the migrations. But, they never run npm install. I must be missing something. How do the Node Modules get into the deployed project?