If a MacBook pro’s internal SSD fails, will I be able to boot and run it by using an external ssd drive?

The Mac has a substantial number of tools that are available even if the boot drive has failed so badly as to be no longer present. Most are part of Recovery mode, but a few may fall through to, or require, Internet Recovery. (which itself requires a working Internet connection.)

Use macOS Recovery on a Mac with Apple silicon - Apple Support

Right out of the box, changing the startup drive is impossible without first discussing your needs with Startup Security Utility, to allow external drives to become bootable. THIS MAY NOT BE POSSIBLE after a drive failure, unless set up ahead of time.

Change startup disk security settings on a Mac with Apple silicon - Apple Support

Note that this procedure requires you to choose a drive AND enter an admin password. That may not be possible after a drive failure. If it were possible to change from default maximum security after a drive failure, your Mac could be more vulnerable to being re-purposed after a theft.

Pretty much as Grant Bennet-Alder says. I would add one concept to the advice of the other gurus here. A current, properly maintained and updated Time Machine Backup. Assume your device will fail, and plan for how you will recover. Don't forget, depending how you assess the risks and potential loss that might also include an off-site storage either in the Cloud, or as a physical backup rotated out to a different location.

In my experience the probability of a SSD storage failing is extremely low to the point it almost becomes impossible to predict. If they survive initial production testing they will (with care) usually be good for many years. The risk of any individual critical component in any Apple MacBook failing within its support lifetime and not being repairable is also very low. Even to the point that your new SSD did fail, and considering they are soldered to the logic board it would be possible to get the logic board replaced. If the device is no longer supported and spares not available it is probably so old there will be other parts on the point of old age, and it would be more economic to consider replacement. Whichever route at this time a backup will get you out of trouble.

The much greater risk is for something Very Bad to happen to the whole MacBook, whether that is catastrophic physical damage like a fire or flood, allowed to descend the stairs on its own, theft, or any of the other risks of day to day life. You can recover from this only if you have deployed a proper backup strategy designed around your needs.

From what I have read online from several reputable sources and from a couple of posts on these forums where the users had erased the entire internal SSD, it is not possible to boot the Apple Silicon Macs if the internal SSD fails or is completely erased. Some critical components are stored on the internal SSD which seems to be needed even for booting into the special startup mode to access Recovery Mode. In order to boot from an external drive requires accessing this special startup mode.

See this article which has some technical details about the new drive layout of the Apple Silicon Macs and how the One True Recovery Mode works (1TR):

https://eclecticlight.co/2021/01/14/m1-macs-radically-change-boot-and-recovery/

here's a notable pull quote from that article:

This ingenious new boot process does have consequences, though. Failure of internal storage means failure of the whole Mac, which can’t then boot from an external disk, which lacks the essential iSC and can’t provide 1TR either. I think this is already true for Macs with T2 chips, with their single security policy, rather than one for each bootable operating system as in the M1. I suspect it’s also, in part at least, responsible for the lack of an Internet Recovery Mode in M1 Macs.

It should be possible. Just hold down the power button on startup. The firmware would need to be intact.

Change your Mac startup disk - Apple Support

On a Mac with Apple silicon: Press and hold the power button until “Loading startup options” appears.

Apple-silicon Macs are very protective of the User data. Although a revive operation may suffice in some cases, in certain rather dire cases of malfunction, the only way forward is a firmware RESTORE. This erases all user data in the process. Device Firmware Update mode is used to re-write private portions of "the" disk, presumably the Internal disk [only].

It is not clear from the articles, but they suggest that the internal disk is the only suitable disk, and that it MUST be functioning well enough to support this operation.

Revive or restore a Mac with Apple silicon using Apple Configurator - Apple Support

I don’t think these answers are correct. If your internal ssd fails completely on an apple silicon Mac, you can no longer boot to an external drive. Unless you are willing to pay for a very expensive repair, you just need to buy a new Mac.

Hi,

As far as I know, Apple Silicon Macs are not able to start up from external devices.

Also note that I think it's very rare that the Macs internal SSD would fails.