why no cliiping distortion when multiple tracks played together

i have two identical tone tracks with max amplitude = 1. when i increase the gain over 0db of a single track i get clipping or distortion. also when i mix the channels additively without normalizing again i get clipping, as expected. what i do not understand is how come i do not get such a distortion when i just play to tracks in parallel (each at 0db). is the audio driver or sound card doing some kind of normalization when it plays multiple streams in parallel, or does the addition happen after conversion to analog and the speaker has some headroom? if it is the first option - i would like to understand which component is doing exactly what normalization and how to control it. if it is the second option - then i would like to understand why would anyone want to ever mix tracks together in the first place, rather than keep multiple separate tracks playing in parallel and does avoiding clipping issues while retaining loudness. is there some limitation on the number of parallel streams? i am on windows 10 with a builtin realtek sound card.

If both tracks are identical and they hit 0dB you WILL get clipping if you mix at full-volume.

what i do not understand is how come i do not get such a distortion when i just play to tracks in parallel (each at 0db).

There are a few possibilities…

You don’t always hear distortion when there is clipping.

Audacity itself uses floating-point so there is virtually no limit and your “data” can go over 0dB. If your playback volume is low enough, your DAC won’t clip. If you export as floating-point WAV, it also has no limits. MP3 can also go over 0dB. It seems to have a limit but I don’t know what it is and it may not be a fixed-predictable limit. So you can make a file that goes over 0dB without clipping and if you play it back at low volume you won’t get any clipping. (But, it’s 'bad practice" to “produce” a file that goes over 0dB.)

So… One solution is to export the mix as 32-bit float WAV. Then re-open that file and run the Normalize or Amplify effect to bring the levels down (if necessary) before exporting to your final-desired format.

some kind of normalization

Normalization is a linear volume adjustment and it doesn’t fix a clipped/distorted wave shape.