Not the same as 1*2*3*4*5*6*7*8*9*10*11*12*13*14*15*16*17*18*19*20.
what is....

If you ask Google, it gives the wrong answer ... 7.96759199e20

I used one of the arbitrary precision calculators online.
(I needed a different special calculator in connection with fermat's-last-pizza.)

Which part is unclear? Arbitrary precision means that the maximum number of digits that the calculator can work to is not fixed in advance. If you multiply a 20-digit number by another 20-digit number, it can produce an exact answer of up to 40 significant digits. If you multiply two 200-digit numbers, it can produce an exact answer with up to 400 digits.

"There is a greater variety of pizzas than there are chess positions. (I have discovered a wonderful proof for this, but the margin is not big enough for me to write it.)"
Precisely calculating the number of pizza toppings versus the number of chess positions was beyond the wit of the one built into Windows.
1*2*3*4*5*6*7*8*9*10*11*1213*141516*17*18*19*20