Front to Back and Inside Out : Bargeboards
Turning vanes, the vertical plates which are usually curved, mounted on either side of the chassis between the front wheels and side-pod intake first appeared on cars in the early 1990's. In the late 1990's and early 2000's they began to appear more complex, often more than one element on the car, and were put in more places such as under the nose cone and at the edges of the side-pods. The 2009 regulation change made the turning vanes simpler and restricted their positioning, further implemented by the 2019 effort to make overtaking easier which required them to be lowered.
Turning vanes, are called so because they turn airflow around the car to reduce drag from the exposed side-pods. They have many functions, but the primary one is to control the turbulent wake from the front wheels and to grab some air from the front wing and the under chassis area. This turns the airflow to the side-pod, which aids cooling, floor and diffuser (which are good for downforce).
Multiple elements are now common
The aerofoils help direct airflow either around the car, which means most of the air sticks to the sidepods until the diffuser due to the Coanda Effect (which states fluids generally will stay on surfaces) and increase downforce, under the car to again flow to the diffuser (usually further back as it's nearer to the diffuser, too early and there's lift which reduces downforce), or rarely on top of the car to increase pressure and have the air push down onto it.
Each design is specific to the car as each vehicle has it's own airflow pattern; only some turning vanes and winglets will help increase downforce, otherwise it's out of sync and is just a drag-contraption.
Directing airflow also helps cool the internal units, as more air intake means more oxygen. Oxygen boosts turbocharging and the higher the concentration the cooler the air, allowing the engine to work better. On some tracks the bargeboard can be modified to favour downforce, potentially cooler circuits, while tracks like Mexico which have less oxygen in the air and are hotter need more cooling. In this case the parts can be directed more towards the side pod intake, instead of under/over the car.
But... There are negatives
Despite all the apparent benefits that bargeboards bring, they cause a big percentage of the overall turbulent wake produced by cars. The many vortices and elements massively disrupt airflow by supplying it to the diffuser or rear wing which in effect throws up the air, meaning there's less of it for the car behind. Aerodynamics rely on, well, air to work so if there's less of it then the result is poor downforce. This is on top of what's already created by the front/rear wing and other devices.
This is why the 2021 regulations are getting rid of them to rely almost completely on what I call 'green aerodynamics' with the ground effect coming back. This is also why the times are expected to take a fall, as bargeboards contribute to so much performance.