I'm not sure if I'm downloading komplete 15 properly to my external hd, since theres no "komplete 15" download and its individual downloads, I'm not sure what to download
Perhaps this will be of help ? : What Types of Products Are Included in Komplete 15 ?
You will want to install that which in Section "Komplete Instruments and Libraries" lists as both standalone / plug-in . And the three plugin only synths : Massive X , Knifonium and BX_Oberhausen. All these standalone / plug-in must be installed to default location on system drive , which is set in Native Access as Application location. Same goes for what other plugins you want installed ,e.g. effects plugins.
Then the Content (Instruments , libraries and expansions) of your choice can then be installed to the external drive , which location is set as Content Location in Native Access.
Otherwise then the e.g. the prerequisite for using Kontakt libraries is that you have Kontakt 8 installed , and prerequisite for using Reaktor Instruments is to have Reaktor 6 installed. Expansions rely on that you have the proper standalone apps / plug-ins installed to use them (on the page linked to they are referred to as 'various*' since some expansion either contains more types of elements or elements that can be used by more standalone apps / plug-ins) . You probably will also want to have Komplete Kontrol app installed.
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Komplete 15 is the name of the bundle, not a product. You should see all the individual products and libraries/expansions available to download in Native Access which is the NI downloader. That's the first thing to install.
You'll then need to use NA to download the applications (Kontakt, Massive, FM8 etc), then whatever expansions/libraries you need.
If you have the Content Location in Preferences set to a folder on your external drive it should automatically download things to that folder. Don't change the Application Location as that can confuse things.