during the spring & autumn period, the chinese liked to use a type of joint task force revolving around a battle chariot. it includes archers, spearman, swordsman, halberdiers and heavy armored charioteers. it can be used as a mobile battle platform and it was proven to be versitile and effective in melee and skirmish *(this was based on availablity of battle chariots or light chariots, rest of the infantry would be arranged otherwise). however, after the introduction of crossbows, the heavy clusters of joint units became easy targets for crossbowman who could use drilled volleys. this led to the decline in chariot warfare and also along with the advancement in military strategy, shifted the chinese forces to become more lighter and mobile. crossbows at that time could out range of bows(200yards), and it could penetrate shields and heavy armor, so it is very different to arrows. however, heavy armored elite units were still existed
as to note the qin conquests, look up "bai qi". he is the commander who defeated the alliance of wei (250,000) with 100,000 of his own, the person who won zhangping (400,000+ enemies slain) and he was also the general responsible for turning the chu counter attack into a full retreat, and he captured the capital of chu. he then decimated the country and was only called back by his king because the king wanted to keep last of chu as a buffer zone. he is rated as the best general of his time, and one of the most successful generals of all times. this is not to belittle all the efforts generations of kings made to make qin a powerful kingdom, but if you'll notice qin loses quite a few of its battles without bai qi and wang jian(as bai qi defeated all of the foes, wang jian finished up the job by unifyng china). and its not the hyped up military like you always hear about. wei has alot of professional and elite soldiers before their defeat, han had the best bows and swords, steel infact. zhao had excellent cav due to reforms, and chu had a huge population, and was able to mobolise 1m conscripts, although chu was the only kingdom where slavery was still legal. they are all by no means a walk over.
edit: many of the acts of attritions were basically done by bai qi as well, and he has a bad name due to that. he gave the orders to continue to pursue and kill after defeating wei-han alliance, he gave the orders to kill the 400,000 prisoners, and he also flooded a city in his chu campaign (mind u, he was cut off from supply, behind enemy lines and enemies out numbered him atlest 10-1)