Look at the bottom of your foot. The metatarsal for your pinkie toe is supported by a pad of flesh that it theory can touch the ground almost all the way from your heel to the ball of the foot. With fallen arches the ankle rolls in, the mid foot starts to touch the ground and the strain on the ligaments of the arch and the inside of the ankle are increased.
On the hand, that is the same side you would use to 'karate chop' someone. Ie, using your hand like a knife. The knife edge. Some people use the same term for the foot.
In the 80s they used to put giant insoles into your shoes and it made walking painful until 1) the insole compressed or 2) you twisted your ankle out so you wouldn't put all your weight on the insole. But it turns out it's better for people to manage this on their own and you have shoes like Vans that hardly have an insole at all.
One thing I've found personally but not in the literature, if the shoe is tight around the ball of your foot, you are not going to be able to get your foot posture right. If you need a wider toe box for your feet to be comfortable, solve that problem first before trying to solve the fallen arch problem. Before I threw out my old shoes I would get all sorts of recurring foot pain when I wore the 'uncomfortable' ones.
You can get new shoes, thinner insoles, or lace your shoes differently to get a little more room. I've saved a couple pair of shoes by doing both (insole and relace) to them.