How would I upload and download save data to google drive or dropbox directly from Unity?

That's not as easy as you might t$$anonymous$$nk. Google drive as well as Dropbox have their own unique APIs to access their services. Both require a quite complex authentication service. The question doesn't really apply to Unity answers since it's not a Unity specific question. Unity uses Mono and as such It's just a question how you get it done in .NET / Mono. Google has a detailed tutorial for a console application.

The google API consists of several libraries. If they are 100% managed libraries you should be able to copy them one to one to Unity.

For dropbox it should be pretty much the same.

Take a look at t$$anonymous$$s project on GitHub: https://github.com/Elringus/UnityGoogleDrive

It works with all the major target platforms (including WebGL) and doesn't use platform-specific precompiled libraries.