Yeast turns wort into beer. Everything else on the brew bench is prep. The heritage post at this URL talked about yeast as a friend. That is still the right metaphor if you treat a friend as a living thing rather than as a sachet of dust.
Pitch enough cells for the gravity you actually made. A strong beer needs more yeast than a session ale. Dry yeast wants the rehydration the laboratory describes. Liquid yeast wants to be fresh.
Oxygen at the start helps the culture grow. Oxygen later oxidises beer. That is why people shake a cooled wort on day zero and then leave the lid alone.
Temperature is the Australian plot twist. The published range is not a dare. Hold it. The styles guide explains which families punish a hot ferment.
When gravity is stable, the friend has finished the job. Package as the priming or kegging notes describe. Then wait again. Young beer is yeast shouting. Conditioned beer is yeast after it has sat down.
This is not medical advice about dietary yeast, and it is not a ranking of laboratories. It is a reminder that the organism you cannot see is doing the work you will take credit for.