|But if there is no BIOS then they say that the system will not do anything... Well I don't agree with this.|
When a PC starts up and power is supplied to the CPU it goes through its own initialisation. At the end of this initialization it puts address 0000FFFFH onto the address bus and starts executing code at that address.
The address 0000FFFF is always in the BIOS. It has been like that ever since the IBM PC was made in 1982. It is at the top of end of the original 1Mb of memory that the first IBM PC could address.
These days the first instruction at 0000FFFF is usually a Jump instruction which takes it to another place in the BIOS and continues execution from there starting of with the Power On Self Test. It doesn't matter how much software is on the hard disk or how much software might be stored in memory, if the BIOS isn't there there is no way the CPU can get at it.
If the CPU cannot find any executable code at this address then it just stops, goes into a wait state and does nothing. If the CPU is not executing instructions, then nothing happens. The only way to get the CPU out of this wait state is to press the reset button and it does it all over again until such time as it can find some code to execute.
If you want to understand more about what happens you need to delve into just how the CPU does its stuff. How it interacts with other hardware, particularly memory and how the various registers within the CPU control exactly what the CPU does.