To spread awareness of the Linux ecosystem, the very first "Live CDs" played a crucial role: by taking away most of the fear of overwriting their hard drives, they allowed more people to test and effectively use any spin of the Operating System, and by keeping the filesystem in RAM, have any changes magically disappear on reboot. This has always been possible, RAM constraints aside, because desktop BIOSes tend to support easy booting from external devices.
The situation on Android devices is, however, more complex. Since most consumer ARM devices are not allowed to boot by any drive other than the internal flash storage, the fastboot protocol used on most Android phones solved this by enabling, in most of its implementations, commands to boot a custom kernel (just on unlocked devices). Furthermore, by unofficial means, even iPhones supported by projects like checkra1n could theoretically sideload kernel code from the modified bootloader.
The "network boot" feature has always been, in fact, one of the most requested features to postmarketOS, and maybe one of the points that prevent it, in practice, from getting more widely to the mainstream.