Okay I think I figured it out.
Boosting through enemies works the same way as boosting through tokens did in super stardust.
You get the same points for boosting though them as you would usually get from shooting them, but you also get a bonus, which is the green number. The first enemy you boost through will yield a 10 point bonus, the next one will add 20, and the next 30...40,50,etc.The green number is not the TOTAL bonus, it's just the amount of bonus points from that single enemy. So if you boost through 4 enemies, it'll be 10+20+30+40, which would equal 100. I'm not sure if this bonus score applies to the multiplier, but I'm guessing it does.
Using overdrive works the exact same way, except with 100's.
I was thinking that the green number was the total bonus you got from that boost or overdrive, but it's not. Perhaps at the end of an overdrive or boost, there should be yellow numbers that shows you the total bonus you got. So with the 10+20+30+40 example I gave above, the green number would be 40, but it should go away and a yellow one should come up with bigger font and say 100. It would be kinda like getting group kills in super stardust, where killing a group of enemies would double all of the points for that group and it would come up with green and bigger font to let you know your total bonus.
I figured this out by watching gameplay on youtube and pausing it and using windows calculator
I hope it's right...
edit: I also figured out how the human scoring works.
they start out at 10k the next one will be 20k, but if you get it in when there's blue arrows, it'll be 60k(50k more than the last one). Then you would think the next one would be 30k, but it would actually be 70k, because the 50k ones actually affect the next ones. So if you save 4 in a short period of time it'll be 10k+60k+110k+160k. That's a lot of points!! But if you lose a human, they start from 10k again.