What Happens if My External Startup Drive Fails? (Mac Mini M4)

Hey everyone, I’ve got a Mac Mini M4 that I’ve set up to boot from an external SSD. Everything has been running smoothly until I started thinking. what happens if that external drive fails? To have tried, removing the external drive and tried booting up, hoping it would fall back to the stock SSD. Unfortunately, it didn’t. I also tried holding the power button during startup, but nothing came up on the screen. If my external drive ever becomes faulty, what’s the workaround? How do I: 1. Boot from the stock SSD again? 2. Set another external drive as the new boot disk? Any advice or steps to prepare for this scenario would be greatly appreciated! Thanks in advance. 🙏