There are a few interesting takeaways from this Rene Herse article on the aerodynamics of gravel bikes/riding. If you just want the TLDR; body position is where it’s at (duh). I did enjoy that they corroborated my assertion that aero gravel wheels aren’t a great idea :). But I would like to see some independent corroboration of the claim that a handlebar bag is more aerodynamic. They claim a small gain, probably as a fairing, but if it is true it might be one of those things that’s highly dependent on the rider and bike used for testing. It’s definitely counter-intuitive.
The biggest part of a bike’s wind resistance is caused by the rider. Unfortunately, there isn’t much you can do with respect to the airflow around bike and rider. There are too many exposed elements to keep the airflow attached and flowing smoothly. When the air detaches, it becomes turbulent, which greatly increases drag. That’s also why aero frame tubes may work in the wind tunnel (without a rider), but once the rider’s legs are spinning right next to them, the turbulent airflow no longer sees much benefit from the shaped tubes.