For metallic dragons, the shapeshifting is detailed under the Actions section of their stat block.
Note that the age at which a dragon can shapeshift depends on their type: brass and copper dragons can only shapeshift once they reach the ancient category (801+ years), while bronze, gold, and silver dragons can shapeshift starting from adulthood (101+ years).
The stat block entry for this ability is the same for all dragons that possess it:
The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon's choice).
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.
So, with that in mind, to answer your questions:
1. How does one determine the stats of its alternate form?
This is explicitly detailed in the second paragraph of the Change Shape ability.
2. Can it have more than one alternative form?
Yes, it can polymorph into any humanoid or beast with a challange rating no higher than its own (mind that the CR depends on the dragon's age!).
3. How long does the transformation take? Can it do this in combat?
The transformation is a full action, and it can be taken in combat.
4. Can chromatic dragons do this as well?
No, nowhere in the Monster Manual is it indicated that chromatic dragons can do this, neither in flavor text nor in stat blocks.