It is said that 8051 has an on-chip oscillator but requires external clock to run it. What may be the internal circuit for on-chip oscillator, how it runs after connecting crystal oscillator? Also what is the purpose of external capacitor with crystal oscillator?
"this is not true for most modern derivatives"
Which is why I started my post with the sentence "The 12-clock feature of the original 8051 isn't to remove any noise." (emphasis on "original" added now)
Today, transistors are almost free. With a design where the inner transistors are very small and just the I/O transistors are scaled large for protection and current drive capability, the transistors consumes very little space and requires very little power. So no longer a need for long sequence machines just to process a single instruction.
Smaller transistors also means less capacitance and shorter travel distance for the signals, so there are less need for wait states for data to travel around the chips.