As far as fast cash goes, dismantle your military. Sell off the assets. Unless you're WL3, on Fearless Blue, or are trying to advance in war level you don't need them. Next, ask the Security Council for money. Not sure what planet you're on, but each planet has a security council that is allowed to release funds to developing countries. It's a few Trillion, I forget the exact amount. You'd need to ask on that planets page, or you can Direct message them. You can check who the current council is on your planet's security council's page.
The best way to make money is to have profitable countries and a lot of them. It is easily possible to have a single country of 100 million people make at least 100b every month. Get your welfare index up to 120. Make sure social security payments are at least 85%. Set supply quality of state owned corporations higher. Set salaries higher. Upgrade corporations. Make sure you're buildi