What happens when battery capacity reaches 0?

Most people have their batteries replaced long before the Maximum Capacity would even get anywhere near zero. As batteries wear & age, they don't just lose their capacity, they also become more unstable where the laptop will suddenly power off even when you may still have a significant charge on the battery remaining.

Just ignore the battery health information and just use your laptop. When the battery is no longer performing to your expectations, then check the Battery Condition reported by macOS. If the Battery Condition is "Service Recommended" and you dissatisfied with the battery's performance, then it is time to replace the battery. If the battery is performing to your expectations, then ignore the "Service Recommended" condition until you have an issue.

Batteries inevitably age and fail.

The battery health value is an estimate of the state of the battery.

Batteries typically become non-useful well before complete failure, too.

Various resources and documentation are available:

• About battery health management in Mac laptops - Apple Support

• Determine battery cycle count for Mac laptops - Apple Support

As a battery ages it eventually becomes too worn to be reliable, and the Mac will throttle its performance in an increasingly desperate attempt to remain functional. Nothing will work well. Normally it takes many years for it to get that bad. It is very likely that your Mac will be significantly outdated by then. You will want to upgrade to the M15 processor or whatever Apple has in store.

Case in point, I routinely use a MacBook Air that is now on its second battery. It has recently been indicating "Service Required" but it still goes five or six hours on a full charge. That Mac is now close to fifteen years old. I am in no hurry to replace it.

This MBA's initial acquisition cost plus a replacement battery over the course of fifteen years with zero residual value equates to a little over $100 a year. It's still working as well as it did when it was new. Most people spend a lot more than that on coffee. Macs are the most inexpensive computers on the planet.