Just curious why C is used vs just putting the constant/peak amps a pack/cell can provide and what the peak amps they can be charged at. Tried googling it, but just finding a ton of "how to calculate" and "what c rating means" information.

It’s basically a marketing tool at this point. There’s no standard

C rating is a complete wish of A batteries performance.. C rate needs to be a factor when finding amps battery should supply.. and a charge rate is normally listed in manual of battery but others have info on there sites..I always do the 1c charge rate.ie 6000mah battery i charge at 6 amps. sure I go up to 10 but I never do 30 amps like some say they do

C is relative to the amount of current the battery is rated to handle, generally most brands will post several numbers related to "C rating"

100/60C will tell you that the pack can handle 100C "Burst" for a short period of time (typically a few seconds) and 60C for continuous.... some brands define continuous as a 30 sec burst, which really isn't continuous... facepalm!

So if you are running a 5000mAh pack, that converts to 5Ah, you simply multiply the C rating to the pack capacity to see how much current it can support, i.e. 5Ah x 60C = 300A of continuous current, but lets get real, there is no such thing as any 1/10 car that can continuously pull 300A of current, that would fully drain the battery in 60C / 60 sec = 1 min, that would be insane to expect a pack to be able to drain in just 1 minute without catching on fire!

If the brand doesn't specify what charge rate then you assume 1C, so for a 5Ah pack you set the charger to 5A charge rate.
Many brands of batteries will support up to 5C rates, so 5Ah x 5C = 25A though many racers will push this limit and charge their packs at 40A, more info here, skip to 6 min into the video: