HDR mode consecutively takes pictures at different exposures such as normal light that allows adjustment for the darkest and brightest details, then taking advantage of its A14 Bionic chip that is able to apply advanced computational mathematics to mix the different shots and produce a stunning photographic effect.
For a person who shoots outdoors, the iPhone 12 Pro Max provides a technology that is called Smart HDR 3. Older HDR software was dreadful, at least when compared with the Smart HDR3, which made images look manipulated.
Technically Defining HDR
HDR 3 uses machine learning (ML) technology of the real-world to train its neural engine, a component on its microprocessor A14 Bionic. When you use your iPhone to consistently take pictures of a particular type of scene, after a while it starts to recognize specific scenes and then automatically create the right exposure for each one. So, when you go on your travels, those of mountains, clouds, snow become recognized easily with the help of HDR3.
Putting it in another way, you can shoot something which you have shot before, and it will know by itself what exposure would be better without the need for toning.
There is something called Deep Fusion also, which means instead of just combining images and providing final output. It selects the best pixels for the colors, sharpness, highlights, and shadows before telling the engine to combine those pixels into the final photograph.