I would imagine life on Earth with a guardian present is basically like the whole planet has been thrust into the Stone Age. There's likely been a mass die off in response to all the infrastructure collapsing.
I imagine that it's not that bad off. After all, it's only human infrastructure. Following what would obviously be a period of attempted resistance against the Guardians and the Created, in which there would most likely be pinpoint strikes against military and government sites to quell any sort of uprising. Given that Evil Cortana and the Created are all based from Humanity, there would obviously be a little more leeway here than on former Covenant worlds in terms of 'obey or die'. Once all active resistance was crushed and the civilian population showed no signs of continuing hostilities, then I see the Created allowing for human civilization to resume under heavy monitoring and control, but otherwise 'normal'.
Power plants being allowed to come back online, industrial outputs resuming along with commercial goods and services. Broadcast entertainment programs. The Created would have no real reason to do anything other than monitor all communication on the planet and take precision action against individuals that were stepping out of line, or perhaps would have stepped out of line in the future, up to and including isolating the individual and stripping them of goods and property, or even outright execution if said individual was combative and unwilling to conform to the desired behavior the Created wanted.
You'd also be looking at mass re-education and indoctrination of the planetary population towards favoring the Created viewpoint. Harder with adults, but starting with younger children and teaching the that 'The Created are your friends. The Created are making a better world for you and your family. Do not resist The Created. Resisting is Futile", etc. Eventually the goal would be to more or less 'domesticate' Humanity into accepting it's new role under the Created's guidance and leadership.
That being said, yes, you'd be looking at planetwide damage, most cities would probably get blapped. The global economy crashing to a halt, large swathes of the human population being wiped out in the early stage of resistance....Certainly BAD things would happen and Humanity would be in a rough state until the Created stepped in and helped the pacified Humans begin rebuilding all the infrastructure and other damage caused...But I honestly don't think it would be all that horrible in the long run.
Oppressive? Certainly, but not to the point where someone sneezes and they get put against a wall in front of a line of Armigers. If you follow the rules and uphold the Imperial Peace, you'd be fine.