I am following a Microservices course that creates a free NMP account and pushes a package he writes through the course into it and imports this package in other related files of the application like a common NPM module/package.
He also mentions there are other options like using Github or some local Git/version controller, etc.
As I am trying to make a commercial project for my own startup, I am curious to know what is a good/affordable option for a startup company in this situation? Is is still a good idea to use NPM free public account to push some packages and use it through our application?
I think that if you are creating a module that can be useful to the community or part of it, NPM is an interesting option to keep your packages.
If you're uploading not usable packages, but your whole application to npm, then that's messed up, NPM is a package registry. Github, Gitlab, and tools alike should be a more suitable option since those are general-purpose code repositories, not package registries.
Also, be aware that the packages you upload on a free npm account are available to any NPM user. see: NPM Private Packages
From my previous experience, both GitLab and Github offer good free small professional accounts that may be suitable for your case.