From what I can tell, Internet Recovery (which downloaded a recovery OS) is gone and replaced by 1TR which is stored on a hidden flash partition. 1TR is capable

From what I can tell, Internet Recovery (which downloaded a recovery OS) is gone and replaced by 1TR which is stored on a hidden flash partition. 1TR is capable of downloading the OS and installing it, so in a way it can be considered the replacement for Internet Recovery. While architecturally simpler (and probably more secure by allowing the network stack to be removed from the low-level boot infrastructure?), the disadvantage is that you can mess up the 1TR partition. If you do, the only way to recover the machine is to do a DFU restore from another Mac via Apple Configurator. That's not terribly convenient if you don't have another Mac around. (An Apple Store can DFU restore for you, but last I checked Apple Stores are appointment-only due to COVID and it's almost impossible to get an appointment there these days.)