how the CPU start by execution stored in motherboards flash memory chip
There's a fairly detailed answer on stackoverflow here. 

Bottom line, is that the hardware has to decode the address space such that the BIOS ROM appears at the appropriate address. 

Here, for example, is the original IBM PC memory map from an online book. Things have moved on since then, but this should give you an idea. The actual decoding logic is the equivalent of gates, probably implemented in some other form (used to be PLDs, but probably a side job of more complex support chips now).