The Purism repositories are at https://source.puri.sm
You can find existing repositories and bugs/feature requests without an account. In order to file new bugs or comment, you will need an account and that requires manual intervention by a moderator due to earlier spam problems on that site.
Knowing what is included in an update depends on the level you are interested in. If you use the PureOS store for your updates, tapping on the listed update will show some information about included updates. You could instead use something like "apt changelog " to see change logs. Those are both pretty dependent on the developer including the changes, I think. If you find the repository for the package in which you are interested, you might be able to find the commits included in the release but I’ve sometimes found associating a package version in a package manager with a commit/branch/tag in a repository to be… hit-or-miss.
There is also a staging area for packages in PureOS that you can monitor, but I can’t remember how to get to that right now.