Stefaan,
This is not true, your SID and STAR created in SimBrief does carry across into Charts. Just make sure you use the Import → From Simbrief function in Navigraph Charts - do not manually copy and paste routes from Simbrief.
Please check this video on how to import a flight som SimBrief to Navigraph:
When you have that flight imported in Navigraph Charts you can export it to MSFS, again including both SID and STAR. We have a video on that too: 16. Export Flight Plans from Navigraph Charts to Microsoft Flight Simulator 2020
You can also load the SB flight plan in FS2020 as you noted, I had expected SID and STAR to be imported too but if it doesn’t then export it from Navigraph as described above.
Airways are normally used in airspaces where such exist, but in many parts of Europe for example, airspaces are transitioning to “Free Route Airspace” (FRA) where there are no airways. As for the different routing, we have different optimization algorithms, but both are likely valid.
We always update the data on AIRAC release day, as we have in the past 15 years, so our data is guaranteed to be current. Furthermore, the data is not the same - we source our data from Jeppesen, which generally has the widest and most accurate coverage of all navigational databases. There are quite a number of procedures added/corrected by our data. Another benefit is that our data also ensures the same naming convention for terminal waypoints, so that the data in the sim perfectly matches our charts.
Cheers,
Stephen