On Mass Relays
On the topic of mass relays and transiting them - we can observe a manner of macro-scale uncertainty principle at work.
Heisenberg uncertainty principle states that we cannot predict all values of a particle perfectly at the same time. There is a limit to precision of other variables, if we know one for certain.
Mass relays appear to have a mechanism reminiscent of the uncertainty principle in that they have a finite amount of detail assignable to transiting mass and where the transiting mass arrives.
A communications buoy can transmit a laser beam at very high accuracy to another buoy on the other side of a relay, as photons are light. The available precision can be dedicated to aiming the laser beam to its destination, another buoy with high precision.
Whereas coordinating a massive fleet of tens of warships, the available precision goes into tracking the thousands and thousands of (kilo)grams of mass, leaving the mass relay deposit the transiting fleet just somewhere near the receiving relay in its host system.
This we know from Joker and the Codex.
One could in theory test the limits of a relay by making it transmit unfathomably large mass (say, the asteroid that destroys Viper Nebula's relay) and see how far out it would come out on the other side. Maybe deep inside the receiving star system or well into its Kuiper belt equivalent.
In addition there must be some manner of quantum entanglement communication between the relays. The lost Mu relay was ejected from its host system by a supernova, yet its counterparts could still link to it.
Viper Nebula relay and the Citadel respectively also had hidden capabilities to access all the primary mass relays to facilitate the Reapers’ plans of massive surprise decapitation strike, so they must have been linked with each and every relay built.