While it existed at the beginning of the war, plate armor really came into its own during the war.

At the battle of Crecy the French basically just mounted their horses, strapped on whatever armor they had, few having actual plate armor, and just charged the English line. Famously it didn't work. The average armor of the knight as well as the armor of the horse was not enough to stop the arrows.

Despite popular belief, the French did learn from their defeats. They quickly realized that plate armor made a knight functionally invincible to the arrows. They also dismounted for their charges. While yes you'd still have cavalry charges, the main thrusts of many of the battles became dismounted knights, slowly walking towards the English line, shrugging off arrows.

Of course the longbow was still effective. Sure the French weren't being cut down by the arrows, however imagine the charge. Walking across a giant field, head bent downwards to protect your eyes as the visor was the only weak spot, arrows fired from bows with roughly a 100 pound draw shattering against you. The fighting condition was amazingly low by the time they'd reach the English. However they were still alive largely due to the tech.

It's important to realize that there was a failure, a further fracturing, of the French state, so although it began as a dispute between the French and English kings over the English monarch owing fealty to the French one for his French possessions, it became much more chaotic, a contest involving various powerful figures, fueled by the ready availability of wandering bands of mercenaries and landless nobles, who would turn into robbers, or routiers, when not otherwise occupied. So, it's not as though there's always simply an English Army vs. a French Army.

One significant change was the growing tendency for taller walls, in bigger cities, as they found small fortifications could be more easily conquered. Simply shutting everyone in and guarding the high walls sometimes was good strategy, waiting until the bandits started to starve and had to go elsewhere.

Jonathan Sumption: The Hundred Year's War