The below videos illustrate the differences between SDR and HDR videos (if viewed on an HDR-capable display) and/or images as their exposure is adjusted. Note the original image becomes overexposed at its natural exposure when viewed in SDR mode.
The SDR and waveform above illustrate how SDR levels quickly hit a signal ‘ceiling’ above which, the image is just rendered as white.
In the HDR video directly (below), the details at full brightness are preserved on most screens but the image may appear slightly darker on SDR screens.
The third video (above) illustrates the same image overexposed 1.8 times yet still retaining detail if viewed on a (good) HDR-capable display. Note the two waveforms above show considerable more headroom. The limit then comes down to display technology, not file format.