In most cases, only one of them maters and the other is negligible: at very small scales QM rules, while at very large scales only GR is important. But in special situations, such as black holes and big bangs, nothing can be neglected and there is no boundary. (AFAIU, IANAP)
Yes, there is an in-between at which Newton laws hold. Too big for QM and too small for GR to make a difference. Fortunately, this is the scale in which we live.
Small point of clarification, the predictions of newtonian laws are a limit in GR. So, technically speaking, newtonian physics is still following GR for that energy regime.
The Planck Scale gives typical dimensions (length, time, energy, mass) for a process where both GR and QM are relevant at the same time. Unfortunately, no experiment will get anywhere close in the foreseeable future.
I thought the Planck scale was way on the QM side of things. My understanding is that the Planck scale is so small we theoretically can never test QM or any other processes below it, making the Planck scale the limits to our understanding of the extremely small.
The plank scale is derived by saying "what length scale appears if we mash together the constants describing gravity and the constants describing QM". One definition of it is "what wavelength photons have enough energy to collapse on themselves and form a black hole". The only thing we know is that when an object is at that or smaller scale we need to consider both QM and GR. It is not "on the QM side of things", because making small wavelengths requires more energy, and more energy is more massive, and mass bends spacetime which requires GR.
No, these objects have nothing to do with the Plank scale. They are described well by the QM and GR we already know and do not show any quantum gravity behavior.
Hawking radiation emitted by a black hole would be a quantum gravity effect. Unfortunately, it is predicted to be many orders of magnitude to weak to be measured.
