OK I'll try to clarify things a bit-Let us start again
AdvOsm has given us the product of his work for free-
From his post #1 you can read all about therm and download links are there.
As for "can these AdvOsm maps be used or opened on any other device than a Garmin?" ,
I have already answered you here post #31
If you have a Windows PC, installing Basecamp will allow you to see the maps, create routes/tracks etc etc etc
Basecamp is available for free from the
Garmin website here. It has not be updated for a loooong time but still works fine- It is NOT an app but a piece of software which needs installing.
Download the .exe, 'unblock and click apply when you
right click on the downloaded file then double click and it will install itself,
As far as your GPSmap 66s there are no pre-installed maps as far as the Garmin site tells me.
It has a max of 16GB of memory but not all of it will be useable as the system takes up some.
It has a micro SD slot so I would suggest you take a SD card, format it
FAT32 ONLY
Create a folder called Garmin
Place all your downloaded AdvOsm maps in there thereby keeping as much memory possible in the internal memory- Garmin units don't like running on minimum memory.
and you're good to go.
The specs of what your 66s are
here and you can download its manual free
here
For batteries... you're on your own!
p.s do NOT download the HD (high Def) version- the O.P has already explained why - it's to do with display quality- the lines will be too thick on your garmin etc.
And
As far as which micro SD card (max 32GB and I use SanDisk ones) to chose and use see Garmin recommendations
here