The onboard Bluetooth device of the Raspberry Pi Zero W and the Raspberry Pi 3 can be disabled by adding dtoverlay=disable-bt to the config.txt file. Wifi is disabled with dtoverlay=disable-wifi.
For raspian installations up until the end of 2019 use dtoverlay=pi3-disable-bt and dtoverlay=pi3-disable-wifi.
The config.txt file is read by the GPU before the ARM CPU and Linux are initialised. It must be located on the first (boot) partition of your SD card alongside bootcode.bin and start.elf. This file is normally accessible as /boot/config.txt from Linux, and must be edited as root. From Windows or OS X it is visible as a file in the only accessible part of the card. If you need to apply some of the config settings below, but you don’t have a config.txt on your boot partition yet, simply create it as a new text file.
Any changes will only take effect after a reboot of your Raspberry.
More boot config options for the Raspberry devices are documented here: https://www.raspberrypi.org/documentation/configuration/config-txt/
Overlays have recently been changed. See here:
https://github.com/raspberrypi/linux/commit/2e2fcd48dbceee231c16e48325600d4d8396583f#diff-f6cd822b204f664348c3ce5c7147aad4
your guidance is still valid as alias remains for backwards compatibility
I will update the post. Thanks.