A similar thread about this issue (that also included that link):
The only question not covered there is:
Project.toml has the list of direct dependencies in the environment (those that you can load with using or import, and can also see in ]status). Installed packages also include indirect dependencies, which are listed in Manifest.toml.
All added packages and their dependencies are installed in .julia/packages, if you want to know the path to them, but you shouldn’t normally have to deal with that. You can leave the job to the Julia package manager.
A short answer to your last question is that you can modify the DEPOT_PATH with environment variables - see the links in the manual and the reference above.
But since you were also asking for tips “for starters”, I’d say that modifying DEPOT_PATH is not the most recommendable exercise for starters. Normally everything should work fine leaving it as it is.
I think that this applies specially if you are using Julia Pro: that’s a distribution of Julia that I think is specially useful for that type of users who don’t want to do any kind of hacking to have an IDE, or deal with broken packages, etc. That kind of user I’m thinking on, doesn’t want to touch the package depot either. (Well, another type of Julia Pro users would be those working in some context that makes worthwhile to pay for Julia Computing’s support, but in that case you could ask them directly.)