More like 5V, 3.3V and 1.8V. These are the voltages used to power electronics / chips.
48V is great if you want to power something like an electric supercharger (which is basically an electric motor) or cut cost on your 1 km long electric harnesses (you can use thinner cables). None of this will apply to Model Y.
Back a ways in time, 48v was looked at for stop/start applications (alternator/ starter) along with electric steering pumps. The problem (one at least) was that all the accessories and bulbs were still 12V. So you have a 48v battery and a 12V with some variety of DC-DC converter.
With LED lighting, all the high output lights have DC-DC converters integrated (LEDs like a constant current supply). So minimal impact to change the supply voltage.
In general, going with a higher vehicle voltage cuts the current, which cuts the wire gauge needed (3 units of AWG is double the cross section). For the same voltage drop, double the voltage cuts wire size/ weight in half (and event with the smaller cable, transmission loss is cut in half). Alternatively, you can supply twice the power with the same wire.
Most vehicles have centralized fuse boxes and body control modules which then have individual circuits to each light or switch (smart box/ dumb loads). But now, if each light or group of lights has a converter anyway, it's a very small step to use a microcontroller to provide the power regulation and on/off control.
Taken to the extreme, all that is needed is one power wire and a pair of wires for networking (vehicle body is the return path). Realistically, fault tolerance is a good thing, so split out loads on a few power cables.
Speaking of those power feeds, remember that fuse box(es)? If the power feeds are DC-DC generated, the converter can provide the current limiting and fault reporting, no need for branch fuses (pre-converter current limiting devices are still reasonable for hardware failures).
As for 5, 3.3, and 1.8, while they will end up being generated for the logic circuits, their regulation requirements do not allow for long wiring runs.