Hi James,
Please see the answers to your queries below:
The Burp Enterprise update will deal with updating the enterprise server and components (everything except the scanner). The Burp Scanner update will update the scanner component - simplified, this just pulls in the latest Burp Pro JAR and updates your Enterprise installation to use it. This is what the latest version of Burp Scanner is. If any additional scan checks (issue definitions) are added, they will be in a scanner update so it is worth always keeping this updated.
The scanner does not automatically update based on any threat or vulnerability feed. Our developers physically write and update the checks as and when appropriate, usually in line with any
research conducted by our security researchers.
You can check out the full list of issues the scanner can detect natively
here. However, you can now write some custom scan checks in Enterprise because extensions are supported as of
2021.8. You may find the resources below helpful in getting started:
You can find all the past releases
here. In terms of the frequency of updates, there is usually one major release per month along with some point releases (e.g. for bug fixes, security updates, etc. there is no exact timeframe on these).
In terms of your final question, the scanner is a major component of both products. When it is updated, it is part of the 'Pro' updates in the release notes.
Hopefully, this is somewhat helpful but let me know if you have any additional questions! :)