Radioactive carbon dating

Let me ask a simple question that I assuming is assumed.How do testers find the initial content of carbon 14? That carbon which was tied up in the object when it was sealed up X number of years ago?I have been told by more than a couple sources that initial C14 MUST be assumed.I know we know the half-life of C14 - 5700 years or close iirc.I know we can find the amount of C14 in an object currently.How then do we know how much C14 (or any other dating isotope) was penned up in an object when it was formed.Is someone assuming?

So if you have the template (such as tree rings) you can carbon date things that are at least as young as the oldest tree?I might can accept that - of course it doesnt help you much past around 3500 years. I hear that some years trees produce a couple rings as well?Glaciars have annual patterns? As reliable as tree rings? I know they have layers that indicate snowfall and melt. I know also that they found World War 2 fighter planes under several hundreds of these layers. So several of them may form in the springs and falls of a year as it snows at night only to melt off during the day?