They don't. I developed and now maintain a weather app for Garmins, and the state of firmware is really bizzare. You can see individual device team merge patches from some kind of core, and then eventually releasing them - so the new bug appears in a simulator, then slowly makes its way to various devices during the next 1-4 months, and then, if you complain loud enough, the same process happens again with a fix.
Garmin really doesn't seem to understand that in this day and age they are supposed to be a software company as much as they are a hardware company.