The https://wiki.openstreetmap.org/wiki/Garmin page on the OSM wiki is basically “write-only documentation” - it’s got lots of irrelevant information but nothing much of use. https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download is slightly better in that it at least links to downloads.
When it comes to actually installing maps, this quite old external page is much better, with the caveat that it suggests https://garmin.openstreetmap.nl/, which is no longer operational, but that does link to https://garmin.bbbike.org/ (already suggested above). It then continues with " Part II: Installing the Maps" which I suspect is what you need.
One caveat I’d add - not all Garmin devices are the same so we can’t guarantee that OSM-based maps will work. If you post details of what is actually on the SD card, we might recognise it as “a regular Garmin device” or “something else”. I’d also make sure that you do not delete any files off the SD card. People sometimes do this to “make some space”, and then find that their satnav does not work at all.
Also, this external forum might have useful info.