Where do you get those 100 digit numbers in the first place? Is there some database or some other computer (server) generating those tasks or what? Sorry if that has been already mentioned but I didn't see that.
They are generated using a hashing algorithm like sha-256. Basically the same way they currently work in bitcoin. Great question. Let me know if you want a more in depth answer.
Yes I would love to have a more in depth answer. I have read some descriptions about sha256 and I didn't see any reason it would produce large numbers that are either primes or semiprimes.
We don't want them to be primes. We want them to be composite so that we can find the winning factor (the factor that meets the difficulty requirements).
Hashing algorithms take any input and produce a large random output. This output can be converted into base-10; normal numbers. Lets say it produces 120 digit number and we want a 102 digit number. we can just truncate the number to the amount of digits we want.