The small blue square at the bottom-right of a selection is the fill handle. Drag it to extend the selected cells. A formula copies with relative references adjusted. Two starting numbers describe a series: 1 and 2 continue as 3, 4, 5; 10 and 20 continue as 30, 40, 50. Include both starting cells before dragging.
Dates work the same way because Sheets stores them as numbers. Two dates a week apart continue weekly. A single starting value may repeat, so check the first few results rather than assuming a sequence was created.
For a formula already at the top of a column, select it and the blank cells below, then press Ctrl+D. The top cell is the source. To enter the same value in several cells, select the range, type the value, and press Ctrl+Enter. These shortcuts work on the current selection.