Short answer: they were similar to 17th century Western European infantry (and also 17th and 18th century Chinese infantry).

In the Qian Han shu (History of the Former Han), we have what appears to be a description of volley shooting from 169BC:

The use of sharp weapons with long and short handles by disciplined companies of armoured soldiers in various combinations, including the drill of crossbow men alternately advancing [to shootJ and retiring [to load]; this is something which the Huns cannot even face.

(as quoted in Needham SCC V5-6, pg 125; brackets in Needham). Given the use of disciplined and drilled infantry armed with hand-to-hand weapons, it would be surprising if crossbowmen (and archers) were not similarly drilled. However, the description is short, and it doesn't tell us about the details. (Andrade (2016) suggests that volley shooting was already in use during the Warring States Period, but notes that the evidence is unclear.)

From the Song Dynasty, we have more detailed descriptions. For example, Zeng Gongliang wrote in 1044 (in Wujing Zongyao):

The crossbow is the most efficient weapon of any, even at distances as small as five feet. The crossbowmen are mustered in separate companies, and when they shoot, nothing can stand in front of them, no [enemy] formation can keep its order. If attacked by cavalry, the crossbowmen will be as solid as a mountain, shooting off such volleys that nothing can remain alive before them. Although the charge may be impetuous it will not reach them. ...

Regarding the method of using the crossbow, it cannot be mixed up with hand-to-hand weapons, and it is beneficial when shot from high ground facing downwards. It only needs to be used so that the men within the formation are loading while the men in the front line of the formation are shooting. As they come forward they use shields to protect their flanks. Thus each in their turn they draw their crossbows and come up; then as soon as they have shot bolts they return again into the formation. Thus the sound of the crossbows is incessant and the enemy can hardly even flee. Therefore we have the following drill -

shooting rank

advancing rank

loading rank.

(as quoted in Needham, pg 122). Needham comments that Zeng Gongliang based part of his writing on a Tang Dynasty source (8th century); this was not new stuff. The illustration accompanying the above description: https://commons.wikimedia.org/wiki/File:1044_Song_dynasty_volley_fire_crossbow_formation.jpg

We have even more detailed descriptions from the late Ming (and also Korean sources, for musketeers, of the time). In Bi Maokang's military manual of 1639, Jun qi tu shuo, we have illustrations showing formations of crossbowmen and musketeers organised for volley fire:

• Crossbowmen: https://commons.wikimedia.org/wiki/File:1639_Ming_crossbow_volley_formation.jpg

• Musketeers: https://commons.wikimedia.org/wiki/File:1639_Ming_musketry_volley_formation.jpg

with the note that the musket and crossbow methods are the same.

These methods are very similar to volley fire as used in 17th century Europe. Later developments such as platoon fire as used in the 18th and 19th centuries in Europe don't appear to have adopted/developed in China; note that before the 19th century modernisation of the Qing army, the Chinese retained the matchlock musket and the pike, rather than switching to flintlock and bayonet.

References:

Andrade, Tonio. The Gunpowder Age: China, Military Innovation, and the Rise of the West in World History, Princeton University Press, 2016.

Needham, Joseph, et al. Science and Civilisation in China, Vol. 5, Chemistry and Chemical Technology, Part 6, Military Technology: Missiles and Sieges. Cambridge: Cambridge University Press, 1994.