Kodi is an app that runs on Android and other platforms.

Android Media boxes are basically the internal chips, boards, memory of tablets and smart phones,
but without their own screen or the function of making phone calls.
They run off mains electricity and plug into a TV screen or monitor via HDMI connections...
They are powerful enough for general net surfing with a choice of browsers
yet can be purchased for as little as 30 quid or less in sales.
I wouldn't trust mine, or most other android devices with sensitive personal financial details like credit card payments and banking,
but they are great for casual surfing and media streaming.

The cheapest ones only have 1 gig of processor emory and older versions of android.
Mine has 2 gig of memory, but is still on the last version of Android 4.

But most if not all Android Media Boxes are factory loaded with Kodi.

Kodi is basically just a media player..

"Fully Loaded Kodi" is a euphemism for Kodi that has been enabled to stream movie & TV content of questionable legality.

The Chromebooks we have are the popular low cost Lenovo notebooks,
got 2 for £99 each in xmas sales..
To be honest I've not tried installing any modifications
as they are just very good internet surfing devices straight out of the box....

I intend using mine mostly for organizing my elderly mothers health & finance, and being able to synch with my mobile phone, PC and tablets etc..
The chrome google office / word processor app is good enough for that..
The wife uses hers for googling whilst watching TV...