All three are really about light
It is not obvious that a spacing calculator, a greenhouse and a lawn belong together, but they are the same question asked three ways: how much light reaches the thing you are growing, and what it costs you to change that.
Spacing rations light between plants. A greenhouse buys warmth at the price of some of it — bubble-wrap lining cuts heat loss by about 45% and transmitted light by about 10%, which is a good trade in December and a bad one in March. A lawn is the case where light is not the constraint at all, and where the limiting factor turns out to be an arithmetic error instead.
Read them in that order if you are planning a plot from scratch. Spacing decides how much you can grow, the greenhouse decides how long you can grow it for, and the lawn is what you do with the ground left over.
Glass and grass have the same trap
Both are places where a published figure gets applied without the conversion it needs.
On a greenhouse, the figure is the temperature lift rather than the target. Holding 5 °C when it is −5 °C outside is a ten-degree lift; holding 10 °C on the same night is fifteen — half as much again, for warmth most overwintering plants do not need. Frost-free costs a fraction of warm, and lining single glass with bubble wrap cuts the loss by about 45% for the price of a roll.
On a lawn, the figure is nitrogen rather than fertiliser. Recommendations are given in grams of actual nitrogen, but bags are sold by total weight with an analysis printed on them. A 20-5-10 product is a fifth nitrogen, so a lawn needing 3 g/m² of N needs 15 g/m² of the bag. Applying the nitrogen figure as a bag weight underfeeds by five times, which is the usual reason a fed lawn looks no different afterwards.