Dynamax lasts 3 turns. Other than HP, no stats are boosted. HP is boosted by 1.5x with no Dynamax candy, and 2x with full dynamax candy. Status moves turn into max guard, which is essentially Protect, except for protecting against max moves, which regular protecting moves don’t. Max Moves BP is based on the base moves power, with a maximum of 140 power. The notable exceptions are max knuckle (fighting) and max ooze (poison) which max out at 90 base power. This is because the secondary effects are so much better than the others, outright boosting an attacking stat of the user and their allies. Gigantamax is a similar phenomenon, which changes the max move of a STAB type to a signature max move, known as a gigantamax move.
Source: experience and https://m.bulbapedia.bulbagarden.net/wiki/Max_Move