We all have heard of "fastboot boot xxxx" command. How does it work when you use it for kernel (fastboot boot boot.img)? Is it simple as this: it boot Android with that kernel if possible or it make that kernel permanent, if not how does it work? Does it just put kernel you specified in RAM instead of one in internal memory? Also what happens if kernel can't boot for some reason does it soft-brick device or just boot existing kernel? And if kernel is ok and it boot how to get back to using kernel from inernal memory?
When you use
fastboot boot FILE.img, the image is downloaded and written into the RAM and than the normal procedure to boot a boot.img is followed. No changes to any partition takes place.
If the image is invalid or cannot be booted into, the boot process automatically falls back to the image in the boot partition. Once the temporary kernel is booted into, it is treated no different from the one stored in the boot partition. You won't get to see "booted kernel" or alike info anywhere in the Android. Once you reboot the device that image gets flushed out of the memory and the one from the boot partition would be loaded and booted into.