How does one earn 8 MILLION credits
Just keep playing.
Credits are all relative. You start off with 1-2 fire stations, so the calls need 1-2 appliances and pay maybe 500 to 1000 credits.
After that, you buy another 2 stations and get calls with 1-4 appliances, that pay maybe 500 to 2000 credits.
Keep expanding and you'll get bigger missions which pay more.
I earn on average 2-3 million a day.
Like others said - prioritise alliance missions. My daily earnings are about 50% personal, 50% alliance. Send a single appliance to an Alliance mission, get it on scene, before it finishes, and you get paid the same as if you sent every appliance. It's the best!