Current Leakage calculations regarding shock hazard. A complete picture?

There is a quantity called current leakage which has always confused me. By reading various definitions, it seems to be the current flowing through the ground circuit of a device (whether this be an actual safety circuit for class I appliances, or the human body in the case of class II appliances). Some sources say it's the current flowing through the insulation, and when a human contacts a conductive part; the current leakage flows through the human.

My question is; how exactly does this quantity come into play when compared to the typical way of thinking about electric shocks (the applied voltage divided by resistance, then refer to a table of safe current limits)

I found this post interesting What is the right way to understand the behaviour of shocking voltage?

Considering all this information, something doesn't feel quite right. The issues I have are the following : 1) Is current leakage simply the maximum current that would pass through the body? (If that's the case, how would you work out the actual current passed through the body?)

2) How is current leakage related to the 'applied voltage' from above? If currentleakage x insulation resistance = 'applied voltage', this would lead to ridiculous results such as dying if you touch plastic.

3) can the body be treated simply as a parallel resistor? I've followed through with some of these calculations and the numbers don't make sense. And current leakage seemed to be made redundant/irrelevant even though it's clearly not due to PAT testing.

4)What ever happened to the law of 'current is the same in series'? When you touch an insulator, you are in series with it (if you are connected to the ground)?