How can Arduino send an RF signal at 433 MHz?
The short answer: It doesn't.
Arduino doesn't send anything close to that high. It can only send a lower frequency (lower bitrate) signal over a 433MHz CARRIER frequency (or any other frequency for that matter) by using another device/module which generates a very high frequency (radio frequency) in order to communicate via radio waves.
The 433MHz in this case is NOT THE RATE OF DATA/BITS, but a frequency of the radio signal over which the data is transmitted/received.
Your question shows a lack of knowledge about the meaning of, and the difference between, processor clock frequencies, speed of processing, data rate, and "carrier" frequencies as the means of radio communication, so you should read and learn more about basic computer processing and its related terms, and about the means and ways of transmitting information over radio waves.
 Basically, the carrier/radio frequency is modulated/changed by the information/data you want to transmit, so it is those changes that represent the information.
 Based on the type of change, there 3 basic types of modulation.
Amplitude Modulation (AM), Frequency Modulation (FM), Phase Modulation (PM).
With AM, the carrier frequency is kept stable/same, while the amplitude/power of the carrier frequency is varied to transmit information.
FM keeps the carrier amplitude the same (at maximum) while the frequency changes up and down a certain amount represent information.
PM is similar to FM, and it carries information by phase-shifting the carrier frequency.