Updated
There are multiple ways to get Units in the game. Scanning flora, fauna, and minerals is the most passive, and will provide you with a 250,000 Unit bonus for scanning all life forms on a planet.
Resources mined and harvested (on and off world) can be sold at traders. Selling salvage can also be lucrative. There are many guides for farming rare items available with various degrees of reward and risk.
Trading across systems can yield higher value for items. If an item is selling for a low price, try checking another system to see if the demand is greater.
Completing missions from NPCs will often reward Units.
Building reputation with NPC factions rewards Units and items, which can be sold for Units.
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All credit goes to Fett_theAssassin at Truetrophies, reposting here for visibility:
This is for having 250,000 Units (money) at the same time. It is NOT cumulative.