what exactly "scanning a system" entails
We have two videos linked in the first post that describe what scanning a system looks like (Search Demonstration and Findables). CMDR Slegnor also has a list of Odyssey signals and when they do (or don't) appear, and a demonstration of signal counting in Odyssey (
link here).
You might wonder why we do X or don't do Y - but it is important that everyone follows the same process. We want to scan things systematically. There can always be follow-up efforts.
does it also involve going to all landable bodies and DSS'ing them?
No, only 8th moons, or planets which have unaccounted signals (see signal counting in the videos - great way to find crashed ships). The reasoning is mostly that mapping every single body would simply take too long. Keep in mind that we are more than four years into this effort.
they want people to use Horizons legacy to scan
This used to be the case, but we allow scanning in Odyssey now. There is more procedural clutter, the UI is worse, and signals are a little different - but otherwise it works. I just haven't updated the docs.
Why even bother mapping anything when searching for something hidden in the game since the beginning?
Eahlstan is correct here, we expect Raxxla to be findable using the tools/systems that were available at the beginning.
I have added Slegnor's signal counting video to the top post.