component selection
Once you have a recipe, you still have to walk the aisles and pick actual ingredients off the shelf — this flour, that brand of butter, eggs of this size. Component selection is the step where the abstract values from your calculations (a 4.7 kilohm resistor here, a 10 microfarad capacitor there) become specific, orderable, real parts with part numbers, prices, and datasheets. The schematic says what you need; component selection decides exactly which manufactured object you will actually buy and solder.
It is far more than matching a number. A real part comes with a tolerance (a 4.7 kilohm resistor at 1 percent could be 4.65 to 4.75 kilohm), a power rating (a 0.25 W resistor will char if you push 0.5 W through it), a voltage rating, a temperature range, a physical package (will it fit, can you hand-solder it), a tempco (how much it drifts with heat), and a price that drops with quantity. You read these from the datasheet, the part's honest spec sheet. A capacitor is not just a capacitance: a ceramic, an electrolytic, and a film cap of the same value behave very differently in leakage, lifetime, voltage rating, and how their value sags with applied voltage.
Good selection is where the design meets reality, and reality has opinions. You pick from values that are actually stocked (the E12 or E24 series, not arbitrary numbers), prefer parts with a second source so one supplier shortage does not stop production, derate so each part runs well inside its limits, and lean toward common, cheap, proven parts over exotic ones. The whole list of chosen parts becomes the bill of materials. The honest rule of thumb: when in doubt, the datasheet beats your intuition — read the actual numbers and the conditions they were measured under, because typical is not guaranteed and 25 degrees C is not your hot enclosure.
Your sum calls for a 1 kilohm resistor carrying 20 mA. Power is I^2 times R = 0.02^2 times 1000 = 0.4 W. A common 0.25 W resistor would overheat, so you select a 0.5 W (or larger) part — and to derate, you might even pick a 1 W part so it runs at under half its rating.
The schematic gave the value; the datasheet decides the power rating you must actually order.
Typical is not guaranteed. Datasheets list a typical column and a min/max column — design to the min/max over your real temperature range, because typical is just the value at one cosy lab condition.