[magicalhippo]

As mentioned it uses PLL. The chip boots using the internal 8MHz oscillator, and you can then enable the external crystal using code.

The clock configuration is actually a bit complex, you can see an example from the STM32CubeMX configuration code generator tool here: https://imgur.com/S9L6U37

HS means high-speed and is used for the system clock and peripherals, while LS is low-speed and is used for the real-time clock. Peripherals have limits, like the ADC clock is shown to be 36MHz, but the maximum is 14MHz. The tool is not complaining because the ADC was not enabled for that project. Otherwise I would have to change the ADC prescaler, which take values like 2, 4, 6 etc. Thus had I enabled the ADC, I would have been forced to use a prescaler of 6, making the ADC clock 12MHz, below maximum.

To get the maximum ADC frequency of 14Mhz, I would have to lower the system clock to 56Mhz.

Other peripherals, like USB, also has some strict limits.