This question already has an answer here:
Header Pins 1, 2, 4, 17 are labled 3V3 and 5V on the schematics but the connections to the BCM and/or regulators are not in their documents. It appears the 5V node is coming from the raw USB/external power, is this true? What about the 3v3 node?
Edit: Note the answer in the duplicate question is not quite correct. The 3v3 comes from the first portion of the PAM2306 (on RPi Zero W for example) which does have and EN1 and EN2 pin that could be switched. So a more accurate question is on the BCM can VDDIO2_1 and VDDIO2_2 be toggled without any ill effect?
Edit2: There are a lot of BCM connections not shown on the schematics. Are there any documents that describe the pin connections for this processor? https://elinux.org/RPi_BCM2835_Pinout shows the labels, but are the functional descriptions all proprietary? Some elude to core or sdram, but it's unclear how they are connected or which voltages are applied.