What are the effects of the Population Adjacent improvement bonus?

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If one builds a colonial hospital and hovers over the tile containing the improvement, the tool tip mentions a "bonus to adjacent improvements":
+2 Level to Population
Unfortunately that's a pretty vague label. The best I can guess is that adjacent improvements like farms will have bonuses to growth?
There are other features and improvements that provide a bonus to population in the same text format, but its not clear at all how the bonus works.
Does anyone have any details behind the game logic of the population bonus?
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How population growth impacts the planet
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