a: I can only speak for the last four years, as I was a Fitbit user before that, but at least during that time the maps were always based on OSM.
b: I don’t know exactly, but there are about 2-4 updates per year. At least with the original TopoActive map.
The big advantage of Garmin devices, however, is that you can easily install your own (free) maps, which are published partly daily, or at least weekly.
By the way, all “our” knowledge about Garmin maps was achieved by…
Never did Garmin tell us how their map format looks like. And we still use an “old” format. Since many years, Garmin’s own maps use a different format which has not yet been reverse engineered so far that we could use it.
I can’t answer “a” because I haven’t used any genuine Garmin brand maps since discovering OSM. Luckily there was a site, now inactive, that supplied Garmin compatible maps for entire countries, or for a smaller region you could manually select. It was a very handy site. I used that site to get weekly downloads of areas I was working in for several years. When that site went dark, I started relying on my own maps compiled with the Java program mkgmap and that’s what I do exclusively now. A side benefit to this is that I can use my own POI symbols and custom colors for ways, water bodies, and landcover.
b) If I’m working in an area and want to see my changes quickly, I merely download the OSM data for that limited area, compile it and then look at it in Basecamp or on my device. Unlike the OSM slippy map, most changes are available only moments after they’re uploaded. There are Internet sites (see above) that provide timely updates for entire regions but those downloads are typically several GB in size and require lots of bandwidth and processing time. And they aren’t as “fresh” as the smaller downloads I mentioned earlier, taking in some cases a week before new data becomes visible.
Another option is to subscribe to OSMAnd’s Live Updates. This service costs about $1 USD per month and allows you to update any number of regions worldwide as often as you wish. For times when I can’t compile a map, when I’m on the road, for example, I just update OSM on my phone to see my recent edits. New data is available using this method almost instantly.
c) IMO, which particular device you use really isn’t important in mapmaking. If you can record a track with it and get it to your computer in some fashion, it can be used to facilitate entering OSM data. I use a small camera to shoot photos of important POIs, highway features, street names, etc., and then once back at my computer and inside JOSM, I correlate those photos with the GPS track using the photo_geotagging tool. By comparing the GPS time (actual time) to the EXIF time on a photo you can derive a correction that will position each photo in its proper position on the track. I just work through the photos one by one, adding the info revealed in each to OSM.
I’ve been doing this for years. It works well and it’s super easy.