Hello Makavelixthadon,
What you have called "mixed audio frequencies" is more commonly called "mixed sample rates".
In typical fashion, Adobe documentation has little to say about this, only:
Resampling, or setting a different rate from the original audio, requires additional processing time and affects the quality.
https://helpx.adobe.com/premiere-pro/using/change-sequence-settings.html
What is meant by this under-explanation is that if you mix sample rates - using clips with sample rates differing from the sample rate of the sequence they are used in - then the audio is re-sampled on-the-fly to match the sequence setting, and this "affects the quality".
[Further resampling will occur if the sequence audio setting does not match the hardware setting.]
In practice, we have no information on the method that Adobe uses to effect this resampling (we are, after all, only users), but it is of usable quality.
Supported sample rates listed here:
https://helpx.adobe.com/premiere-pro/using/importing-digital-audio.html
R.