Process for package updates?

I have had my Librem 13 for a couple of months, so I am still fairly new to this platform. I do not mean any sarcasm or criticism in my question, but how are existing packages in PureOS updated via Debian? I do not see anything in the wiki or this forum that really explains that.

I have seen posts in this forum saying that PureOS is a ‘rolling’ release. I thought that implied that packages were updated as available from Debian test, perhaps after some review. I also understood there were some exceptions, like PureBrowser.

Is there more to the process?

For example, I recently opened a Phabricator task for an update to Thunderbird, which in PureOS is a few minor versions and almost 8 months behind. (Again, I am just trying to understand how this works; I am not complaining.) Based on my understanding, I was a bit surprised this package was so old.