Avionics in not  a Black Art. Quite the contrary, electrons follow a relatively few, well defined rules. The challenge is in the understanding and application of those rules. Most of the rules are simple enough but some are frequently ignored.

If you want to build an electrically "quiet", "low noise" airplane, you would do well to remember the following:

All wires have resistance and therefore ANY current flow will result in a voltage "loss".
There is no such thing as "digital" signals. Consider all signals as analog.
All wires carrying a current are surrounded by a magnetic field that is proportional to the current.
If a moving or expanding magnetic field crosses a wire a current will be induced in the wire. These last two rules outline one way "noise" is passed from one wire to another.
Electricity need a "circuit" to flow. If a signal goes "out" on one wire it MUST come "back".
Cable shields are shields, not conductors (even though they conduct). Shields are to wires as the enclosure is to your radio. Design the system to work without shields then add the shields.
When in doubt, twist. Twisting the "out" and "back" conductors together will encourage their magnetic fields top cancel each other, reducing magnetic noise emission and susceptibility.

Regarding ground, of course there is such a thing. It is the datum from which voltages in the system are measured. If two components have different grounds then the signals that pass between them may not be interpreted correctly. A man with one watch knows the time, a man with two watches is never sure, or as my associate Hamid has said "If you don't believe in one ground, it will smite you.". Since wires that carry current lose voltage in the process (or did you forget already) it is a good idea to have more copper dedicated to the ground connection than the power or signal connections.

If you have dedicated enough copper to connecting all the grounds together, if you twisted conductor pairs together, if you put flyback diodes or transorbs on all relays and solenoid coils, if you routed noisy conductors (strobe power) away from sensitive conductors (intercom power) and if your bus and ground has a low resistance connection to the battery you will likely have an electrically quiet airplane. If you have done a poor job you may need fixes like filters and capacitors. BTW, most airplanes are equipped  with a low ESR, large value capacitor. It is also known as a BATTERY.

All inanimate objects, sooner or later, must respond to reason.

Regards
Brent  Regan