... Not every one of us is familiar with "same power circuit" and how it is physically bordered.
This information is not something you can pick anywhere, it depends on how wiring in particular building is done. You can get that information either from electric installation blueprints or some qualified electrician can help you discover it. Or, if everything else fails, discover it using scientific method publicly known as try and fail
For example, if I have a magnetic circuit breaker kitchen, another for bedroom, another for living room, can I extend the network with such pair from living room to bedroom?
My experience is that it works, but poorly. When I conducted tests with PLC equipment (AV-500), it worked fairly well when used on same electrical circuit (i.e. on one side of magnetic circuit breaker) - speeds reaching around half of theoretical values (i.e. around 250Mbps whereas AV-500 theoretically allows up to 500Mbps). If there were magnetic circuit breakers on the way (probably there will be at least 2 of them), the real speed dropped by an order of magnitude (or two). The same equipment worked nicely over "classical" circuit breakers without notable effect on throughput.
Perhaps unexpected result of a related experiment: when electrical installation features magnetic circuit breakers PLC might actually perform better between outlets on different phases if wires of both phases leading to utilized outlets parallel to each other for a few metres. Obviously inter-wire crosstalk is high enough and magnetic circuit breaker's attenuation big enough to make this difference.
Same phenomenon helps better PLC operation when used on same phase.
If particular wires don't run parallel long enough, then this phenomenon doesn't happen.
Worth to note is that PLC works best on non-stranded wires (the non-flexible ones that are usually used inside walls). If PLC device is plugged to an extension cord, which almost certainly uses stranded wires (for their flexibility), then performance drops (every metre counts). I expect performance might (slightly?) drop when used some plug type converters (e.g. for plugging EU-type device into UK-type outlet).