It's just using the result of the dice roll as digits to compose a number in a base 6 positional numeral system.

Maybe it's not obvious because the formula above is already reduced. Digits in base six can only be 0-5, that's why there's a -1 in 6(da -1). Two digits would give us a result between 0-35, that's why there's no -1 subtracted from db, I already added 1 to make the range 1-36.

You could use two different dice as well, like a d8 and a d6 to make a d48. Or three or more dice with the same number of faces, eg 3 d6 to make a d216 (they have to be distinguishable, in color, size, style...). For more than two different dice the math would get too messy to be practical, I reckon.