Some parts of the chip are intentionally not explained I think. It would expose too much.
For example, in the F4 series there are the ADCxDC1 bits you can turn on in the SYSCFG module that obviously do something to improve ADC accuracy, but they are not described in the reference manual. That just points you to appnote
AN7073. Which is basically a big warning to not use those bits, except in very specific cases...
The F4 is a particularly noisy chip, if you need the ADC you're going to need an HSE and disable the flash prefetcher, so there goes your performance.
The L4 is newer, I hope they learned from the F4 and made it better.
ADC's are tricky to get right. When running the self-calibration you probably don't want anything else running.
With an good enough scope you can probably tell when you're running the FPU on the power rails. This will be amplified with poor power distribution (eg: 2 layer).
Check also this application note listing all possible improvements and mitigations for using the ADC,
AN2834.
@peter-h
The calibration values written to ROM in the factory for the temperature sensor are to provide the accurate gradient for you. You get two temperature with a known Vdda to interpolate around.
The internal reference calibration value is one known point at known Vdda you can then use with the temperature calibrations to put the 50ppm to work.
Using those you can also calculate the Vdda, but not better than the internal reference.
The absolute value of the internal reference still needs external information. When that is given it is actually pretty good.