Hi,
I looked in the Garmin in the menu as you described.
It is not so easy to compare the device and base camp.

1)
I have a European Garmin.
In Base camp I have one Garmin map for Europe.
That Garmin express keeps up to date. Both for the device and for Base Camps

In Base camp this shows as one map
In the GPS is shows as two maps. North and south.
Easy to map the maps

I am happy with the Garmin maps. So I use them.

2)
I have so far loaded extra maps on the SD card.
If I screw up something. I can replace the card. And I do not destroy the device. Even it goes a little bit slower. I might use internal memory later

x)
South America. That is were I try to install extra maps.
(I have plans to travel in SA. And want to use my Garmin also there)
First a map from a memory card that I bought.
One map in Base camp and one map in the device.
(At least that as far as I can understand)
Easy to understand.

Than I have tried with OSM
a) One site where I could click a lot of small regions. So I covered the area that I wanted (Argentina, Chile and Bolivia)
(I can't find the link now. But after selection all regions. The building of the file was placed in a queue. And I could download it later)

b) One site that had SA divided in 4 regions. North, West, South and Brasil

https://www.gmaptool.eu/en/content/s...-topo-routable

c) One site with a specific map for Colombia. For test purpose.

https://alternativaslibres.org/en/downloads.php#Europe

I can see these listed in Base Camp.
But than it becomes tricky.
In the device, I see a list of 50-100 regions. Instead of maps on the level visible in Base Camp. When I look direct at the device.
When using the Windows File Manager. I see in total 151 files. Most of them has only a number as file name. Only the SA map that I bought is a large file with a name

Compare to USA. If Base Camp shows a US map and the device shows 50 states and other regions.

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But now when I know more.

I assume that OSM maps are built up of a lot of maps from small regions.
And the device shows all regions. And it is hard to track each region to a specific download. And to turn on/off means to click all these regions.

And I have no idea if the device uses the SA map, the Country map or the region map. (All are activated)
And I can't find any way to uninstall maps.

I am happy that I have used the SD card. When my experiments are done. I can delete everything with file manager. Or mount a new card and start from scratch. Easier than trying to uninstall. I works. One small step at a time. But it is not easy. And takes time.
But I like my Garmin, when it comes to navigation And a Zumo device is not cheap. So there is only one way. To learn and to get it working
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It is excellent with this type of forum. Where we can share experiences all our try and error.
Thanks
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I have attached 3 files of map lists

1) Device
2) Base camp
3) windows File manager

Try to match !!

Attached Thumbnails