ESP32-C3: why can only 2 of 4 MB flash be used?

ESPHome
1

Why does ESPHome say

HARDWARE: ESP32C3 160MHz, 320KB RAM, 4MB Flash

and then

Flash: [======    ]  58.5% (used 1073158 bytes from 1835008 bytes)

In other words, despite 4 MB Flash it claims it can only use shy of 2 MB. Why is that?

2

You can not update a running system, so the flash is divided in two partitions.
The running system can then update the system in the dormant partition and when it is down the bootloader can reboot to the dormant system.