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human
ELI5: How do Americans (US) pay their taxes and how do they know how much to pay?

Where I live taxes are paid in every item you buy and every service you take. So if you buy an ice cream it's the base price plus taxes. Normal people don't do taxes once a year, only corporations do. 

But I've seen in some tv shows, like The Simpsons, that they have to calculate and send via mail their taxes. 

I imagine you have to pay certain amount depending if you bought a car, or a house, but I can't seem to understand if you pay taxes for a Snickers bar or a Starbucks coffee, and how do you even remember what did you bought in a year.

Edit: Thanks a lot for all the helpful answers! 

So what I saw on The Simpsons as "doing the taxes" was actually (maybe) income taxes that you pay each year, and not sales taxes (that Snickers bar had taxes included in the price). 

With each paycheck your employer take an estimate of what you should pay as income tax, and by the end of the period the IRS ask You how much you should be paying for taxes (not an estimate anymore), so if this number is bigger that the initial estimate then you have to pay that extra ammount, and if it's lower than the estimate then they refurb you that extra money. 

Anyway you should really read any of the other great answers bellow. 

You guys know your stuff really well. I think I now know more about your taxes than I do about mine.
gpt
We don't pay state and federal income taxes on items we purchase. You don't need to remember things like what you paid for your ice cream. Those are local & state *sales* taxes.

The income taxes for most people are fairly simple. You are paid wages. Some of your wages are withheld from you before you get them in order to pre-pay your income taxes. You can opt out of this or determine how much you want withheld.

At the end of the year, you report your gross income and you are allowed to deduct from it certain items. What is left is put through a formula to determine your income tax. From this you subtract what you had withheld from pay throughout the year. Then you can subtract any allowable credits (things that reduce your tax owed as opposed to reducing your taxable income). If the remainder is positive you owe the Gov't more money. If the remainder is negative the Gov't owes you money.

TLDR : we don't pay income tax for purchases. We tally gross income at the end and subtract expenses then determine our income tax.