You are allowed to modify stuff but it is not actually changing the install as is.
This is achieved by different techniques like file system overlays, containerisation, btrfs snapshots and so on.
The idea is to replicate the classical behavior you know from embedded devices that have their core functionality in ROM with even firmware updates only overlayed or modern smartphones: You can modify your system but in the end there’s always the possibilty to “reset to factory settings” as in: the last known working configuration.
In simplified terms:
You are allowed to modify stuff but it is not actually changing the install as is.
This is achieved by different techniques like file system overlays, containerisation, btrfs snapshots and so on.
The idea is to replicate the classical behavior you know from embedded devices that have their core functionality in ROM with even firmware updates only overlayed or modern smartphones: You can modify your system but in the end there’s always the possibilty to “reset to factory settings” as in: the last known working configuration.