Cold brew made from the same beans as a hot cup tastes noticeably less bitter and slightly sweeter. Temperature changes which compounds leave the grounds, not just how fast.
Extraction is selective, not uniform
Ground coffee holds hundreds of soluble compounds, and they do not all dissolve at the same rate. Acids and sugars come out readily, while several bitter compounds need more energy.
Hot water supplies that energy immediately, so a hot brew finishes in minutes and carries the full spread, including the harsher end.
Cold water supplies far less, so the compounds requiring more energy stay largely in the grounds even after many hours of contact.
Time substitutes for heat only partly
Long steeping does compensate for low temperature in terms of total strength, which is why cold brew concentrate can be more caffeinated than an equal volume of drip.
What time cannot do is even out the selectivity. A twelve-hour cold steep still leaves a different compound profile than a three-minute hot pour.
The result is a brew that reads as strong and smooth simultaneously, a combination that seems contradictory until the selectivity is accounted for.
Acidity behaves in two senses
Coffee acidity means both measurable acid content and the bright, tart quality tasters describe. Cold brew usually lowers the perceived brightness more than the actual acid level.
Part of that is the absence of certain volatile acids that form or release under heat, and part is that bitterness and acidity sharpen each other in the mouth.
Remove much of the bitterness and the remaining acidity reads as softer, even where the underlying chemistry has barely moved.
Grind and contact time interact
Finer grounds expose more surface area, which speeds extraction. In cold brew that can tip the balance towards the compounds the method was meant to avoid.
Coarser grounds slow things down, which is why long cold steeps are usually built on a coarse grind rather than the one used for espresso.
Get the pairing wrong and cold brew loses its main advantage, turning muddy and astringent despite never being heated.
Storage changes it further
Cold brew keeps longer than hot coffee partly because it was never exposed to the temperatures that drive rapid oxidation of aromatic compounds.
It still changes in the fridge, growing flatter over several days as the lighter aromatics escape and the heavier ones dominate.
Concentrate diluted only at the moment of serving holds its character longer than a batch diluted at the start, since fewer aromatics sit exposed in a large volume.