Any combination of longer, or more open will lead to greater volumetric efficiency.
More open causes less restriction from the valve because there is more curtain area for the air to flow through.
Longer allows more time for air to flow into the cylinder.
Cams are a bit specific to the type of fuel injection, the manifold, and the combustion chamber setup including valve arrangement.
IF you have fuel injection, and most people do, you may want to keep the intake valve closed for a small portion of the down-stroke to build a bit of vacuum in the cylinder. This is done so that when the intake valve opens, the velocity of the air increases at a high RATE. If you can ACCELERATE the air to high speed quickly you can turn the injector on sooner.
In order to have an excellent distribution of atomized fuel in the air, you don’t want to have the injector (which is digital) spraying into air that is not moving at the correct velocity. If you open the injector too soon the air is not moving fast enough and you will have a mixture that is too rich in the beginning of the intake stroke. If you open the injector too late, after the valve is already open, you will have let in just air which is too lean because it contains no fuel.
Even though the combustion chamber has high swirl characteristics, the resultant mixing is more like a marbled bread than a homogeneous mixture. The marbled bread fuel distribution is the concept behind fuel stratified injection, which is very complicated. FSI is an attempt to get better more even atomization with fuel injectors, either direct or indirect injection.
An ideal cam would wait just the right amount to open the intake valve, accelerating the air at an incredibly high speed, and open the injector is soon as the intake valve was opened.
Of course the valve has to close as well. This further reduces the quality of the mixture. If you wait too long to close the valve you velocity will drop down to a level where the injector is too aggressive for that air velocity and you will once again have a rich mixture. Air velocity as a function of stroke is somewhat of a truncated sine wave with a spike in the beginning.
The initial spike is the sudden rush from the previously mentioned vacuum, which then reduced quickly to a function of the change in volume created by the moving piston. The piston velocity is a sine-like function, with the highest velocity at mid stroke and reducing from there to BDC. The intake valve will usually close somewhere on the bottom of the down-stroke.
Clearly cam theory is far more complicated than understanding Ev, but it is obviously one of the most important factors in cylinder filling.