Not too long ago I was talking about the difference in aspect ratios for cameras. It was a quick talk if I remember correctly and I didn’t dive too far into any of the reasons why we use what we use now.
To remedy that I dug around and found a video explaining all of it for me. It’s geared more towards cinema buffs but much of it is very relevant to still images as well. The article is up on Fstoppers now and if you’re interested I’d suggest taking a look. You’ll find the article here.