How do I make Units efficiently?

One method that's not incredibly fast but reliable (around 1.5m every half an hour) is to simply go to a space port, look at that ports trader, and note down everything with a gold star next to it that they'll buy for +90% or more value than galactic average. Then, go down to where the space ships come in and purchase everything in their inventory that will sell to the space ports seller for twice the value. Rinse and repeat ad infinitum.

You can make two-three million units per hour by crafting bypass chips.

Bypass chips sell for 3575 each, and require only 10 iron and 10 plutonium to craft. This means you can use this method on nearly every planet, with no special equipment or recipes, and without any money currently to your name. It's also very space efficient, as each stack of 500 iron or plutonium will net you 90,000 units worth of chips. This is even better than gold!

For those who doubt that it can really give you this much money, this is how I determined the value: I loaded my game, which I happened to last save by a Trading Post, so I ran over there as my starting point. I then timed how long it took to get 500 plutonium from the trading post, which came out to two minutes. This includes looking around to find a likely location, running over there, avoiding some pesky sentinels, and actually mining it. I had to run a fair bit to find some, and I also mined some aluminium while I was at it, so I feel this is a time you could generally achieve. This comes out to 2.4 seconds/chip (120 seconds/50 chips).

I then timed how long it took to get 500 iron with a mining laser, which was 40 seconds. I didn't add any time for finding iron, as it's pretty much always plentiful near plutonium. I just mined the nearest things, which weren't overly rich in resources, so this should also be a reasonable number to consistently achieve. This is 0.8 seconds per chip.

Next I crafted 16 chips (the amount of inventory space I happened to have open), which took another 40 seconds. I didn't rearrange my inventory at all to make it more efficient, and I'm on the PS4, so you can probably meet or beat this time easily. This adds 2.5 seconds per chip.

Lastly I sold all my chips, which took ten seconds. How many seconds this adds per chip depends on how many you sell at a time, but let's be pessimistic and say you only have ten slots available. This adds one second per chip.

If we add that all up, we see that it takes a total of 2.4 + 0.8 + 2.5 + 1 = 6.7 seconds per chip. There's 3600 seconds in an hour, which gives 3600 / 6.7 = 537 chips an hour. The galactic average for the chip is given as 3,575 units, so presumably you will get that on average if you always sell your chips wherever is convenient. So 537 chips/hr * 3575 units/chip = 1.9 million units/hr. Obviously you will eventually have to travel farther away to get the plutonium, but you could fly to a cave with lots of it and load up before returning to a trading terminal. I think you could sustain this rate if you did nothing but craft bypass chips if you did so on a decent planet and were smart about your process for gathering mats.

This is an average case if you aren't trying to be efficient at all. I ran from a random spot to spots that looked likely for plutonium, then only picked up 500 when there was lots more. If you pick up plutonium as you are walking around and/or clean out large stashes as you find them, you can easily halve that number. If you use grenades to get iron instead of the mining beam, the time is practically 0. (I've gotten 1000 iron in under 10 seconds with grenades.) If you have 20 spots free instead of only ten, then selling them only adds 0.5 seconds per chip instead of one second. This all brings the time to 1.2 + 0 + 2.5 + .5 = 4.2 secs/chip, which is 857 chips/hr. You can also wait until you find a trading terminal that gives you an above average price. The one I happened to sell at gave me 3617 units/chip instead of only 3575. If we work out the math with those values, you get 857 * 6317 = 3.1 million units/hr. Granted, you can't achieve that for 100 hours straight, because it relies on the time walking between nodes being spent on other things. If you're planning to explore anyway, though, then you can't beat the efficiency of picking up every bit of plutonium you see.

My current method, and granted this entirely depends on your planets, has a planet near me which has Gravatino Balls on the ground worth around 25k each.

For me, any planet where the sentinels automatically attack you, has these orbs, designated by a yellow ! when using the scanner.

Also a bonus (depending on how you look at it) is every time you pick up an orb some sentinels come after you, which you can get titanium and neutrino modules for, which also sell for good money.