I'm very familiar with a similar Yamaha AV receiver model, the RX-V673. The information I'm providing below is based only on my work with the RX-V673. The RX-V483 may be slightly different.
The switching power supply in the RX-V673 has only one output: 5.5V. It is capable of providing up to 5A of output current. This is a total of about 25W. There are several secondary voltage regulators, mostly located on the digital/HDMI PC board, which convert from 5.5V to a variety of other rail voltages used by the HDMI and the digital processing IC chips for both video and audio.
The 5.5V output voltage should be present at all times while the unit is receiving AC power, regardless of the main on/off status. Power to the large iron-core transformer (bolted to the main chassis) is supplied only when the relay closes.
"+3.3M" is the 3.3V rail for the "standby" mode. In the RX-V673 this +3.3M rail is regulated by IC85, a tiny SOT23 packaged linear regulator located on the digital processing PC board. This provides power to IC83, the system controller MCU, which is also located on the digital processing PC board. This MCU controls every function and feature of the unit, including the response to pressing buttons on the front panel and responding to IR signals received from the remote control.
As you observed, the +3.3M appears on a connector that goes to the switching power supply PC board. However, the +3.3M comes from IC85, so you will only see it if all of the PC boards are installed and connected to each other. In other words, the +3.3M does NOT originate directly from the switching power supply PC board itself.
I have one "totally dead" and unresponsive RX-V673 unit where both IC83 and IC85 get "warm to the touch" after a few minutes. The switching power supply is working and is producing the expected 5.5V output. However, the +3.3M rail measures only about 2V. I suspect that something damaged IC83, the MCU, and that the 5.5V to 3.3V linear regulator (IC85) is entering its "current limited" overload mode rather than failing violently.
I have two other RX-V673 units which are (now) in working condition. When in the "standby mode" neither IC83 or IC85 get warmer than room temperature.
The faults in my other RX-V673 units were corrected by a proper "factory diagnostic and service menu" reset of the internal protection system. One of my RX-V673 units required a special procedure to "enter the correct model number into the EEPROM" located on the digital processing PC board. Look up "Yamaha AV receiver internal error fix" on YouTube to find the procedure for correcting "no model number" and/or "system error." This procedure is NOT documented in the official Yamaha service manual. I don't know where the YouTube person learned it, but I tried it and it works perfectly. I suspect this "model number" procedure was documented only in the "service bulletins" which are sent to authorized Yamaha repair centers.
I'll soon be posting details of my ongoing project to fix several Yamaha RX-V673 units. I got started on this because my son has owned an RX-V673 for more than 5 years. It suddenly went totally dead 3 weeks ago. This is the unit where I suspect IC83, system controller MCU, has failed. I bought two "for parts or repair only" RX-V673 units (cheap) on ebay to get a fully functional digital PC board, which I then intended to transplant into my son's receiver. But then i found these "parts or repair" units had no hardware faults at all. They required only the "overload reset" and/or the "model number" restoration procedures to get them working again.