Via Command Line Tools

Unity really only needs the command-line tools to build projects for Android. If you prefer, you can install just that package and save disk space. Scroll to the section named Get just the command line tools at the bottom of the downloads page. Select the package for your platform:

This is a ZIP file; uncompress it to a folder and please remember its location. As mentioned, on Windows I like to use D:\Programs\Android\sdk. This will contain a tools subfolder.

The ZIP is only the tools, not the actual SDK. Use the sdkmanager to download the packages you'll need. See https://developer.android.com/studio/command-line/sdkmanager.html for details.

To list the installed and available packages, run sdkmanager --list. You can install multiple packages by listing them in quotes, delimited with a semicolon as follows:

    sdkmanager "platforms;android-25"

As of writing, the minimum Android API levels are as follows (check the current documentation for changes):

Cardboard: API Level 19 (Android 4.4 KitKat)
GearVR: API Level 21 (Android 5.0 Lollipop)
Daydream: API Level 24 (Android 7.0 Nougat)