According to this
You can specify the amount of bitcoin with an (apparently) 16 alphanumeric hex representation. It is stated you must specify the bitcoin you want to send in units of Satoshi.
Here is the quote:
We then write an 8-byte field (64 bit integer) containing the amount we want to redeem from the specified output. I will set this to the total amount available in the output minus a fee of 0.001 BTC (0.999 BTC, or 99900000 Satoshis): 605af40500000000
How did they get
605af40500000000 from '99900000 Satoshis' when that hex code =
6943130077857054720? I can find no conversion calculator online which demonstrates what they are switching it from or to.
I got the hex code from https://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html