Firstly I'd like to point out that I am a very basic user and trying to get my head around a few things.
I have a KML boundary saved from Google Earth. I can then drop this boundary in to QGIS which shows up with CRS of EPSG:4326. I can then open the Google satellite imagery and this confirms that my boundary is in the correct position.
So now I have 2 layers, my boundary in EPSG:4326 and the Google Satellite which shows CRS EPSG:3857.
How are they aligned perfectly if they are on a different CRS?