What are 1e18 1e15 1e12 1e9 etc?

Answer 1

That's just another way to write numbers in scientific notation. The other common way to express scientific notation is 1 X 1018, 1 X 1015, 1 X 1012, etc. All that means is that the number before the exponent is either followed by the number of zeroes in the exponent (if the exponent's positive), or proceeded by the number of zeroes in the exponent (if the exponent's negative).

In the sciences, the first several exponents that are non-zero multiples of 3 (both positive and negative), are given special prefix names. For example, 103 is kilo-, 109 is giga-, and 10-9 is nano-.

Below, I added a related link that gives many more examples of these special prefix names.

Answer 2

These are scientific notations representing large numbers. For example, 1e18 is 1 followed by 18 zeros (1,000,000,000,000,000,000). The "e" indicates that the number is multiplied by 10 raised to the power following the "e".

Answer 3

1000000000000