It probably just has what Garmin calls the “Basemap”. This provides context for areas you don’t have a detailed map for. It’s “useless” in the same way a screwdriver is when you need a hammer.

Don’t buy maps. There are numerous sources of free maps.

The OSM maps are fine. It’s what Garmin provides on the Edge models.

Note that the built-in memory in the Etrex 30 isn’t very large. So, either you are going to keep the map size small or get a microSD card (not larger than 32GB).

The following is a fairly standard source for maps. The Edges require the “Latin” versions (Unicode doesn’t work on them). I suspect it’s the same for the Etrex (but I am not certain). Consider trying a small region first.

https://extract.bbbike.org/

You copy the img file to the “\Garmin” folder on one of the drives that shows up when you plug your device into your computer

Some (old?) devices can only use one map file. For these, the file has to be named “gmapsupp.img”.

For devices that can use multiple maps, the img file can be renamed to anything you like. The name the device displays is data inside the file.

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No, you can’t use your Avenza maps.