Anyone know a way to have the boot manager automatically appear each time an Apple Silicon Mac powers on?
I've installed Monterey and Ventura on my Mac Studio and I would like to be able to select which one to boot whenever I power on the Studio without holding the power button down. i.e. similar to how Open Core provides the Boot Manager whenever it powers up on my Mac Pro, or by holding the Option key on an Intel Mac.
I will be the bearer of bad news: not only is there no way to do this, there probably never will be.
Apple Silicon Macs boot in a profoundly different way from Intel Macs. This is one of the consequences. Using the power button to bring up the boot manager is basically the only user interface function built into the M-series mask ROM responsible for the earliest stage of booting. This mask ROM is impossible to alter (it is truly Read Only Memory), and is tiny. It's part of the M1/M2/etc SoC, so they don't want it to use much area. Perhaps more importantly, the smaller it is, the easier it becomes to thoroughly audit the program in the mask ROM to build extremely high confidence that it has zero bugs.
That extreme simplicity is why its user interface isn't any more sophisticated than checking the status of one button, and it's also why it can't be a button on an external USB/Bluetooth keyboard. You need fairly complicated device drivers to talk to USB/BT keyboards, drivers which couldn't possibly fit and are difficult to fully audit. Checking the status of a single button directly connected to the SoC, on the other hand, is probably one CPU instruction.
By now you might be asking yourself: what's the point of all this worry about bug-free firmware? Fundamentally, the button press is proof to the mask ROM that a human being is physically present and is requesting to enter RecoveryOS (aka the boot picker). Why is that proof important? Well, if only RecoveryOS can change which OS is selected for boot (among other things it can do), and the only way to get into RecoveryOS is to physically push that button during early boot, there is absolutely no way for downloaded malware to fully automate things like installing and rebooting to an alternate hostile OS.
