Ctrl+1 (Mac: Cmd+1) opens Format Cells for the selection. Its Number tab has every format the ribbon has, with the details the ribbon hides: decimal places, the thousands separator, and how negative numbers look (red, in brackets, or both).
Custom shows the format as a code you can edit. Codes have up to four parts separated by semicolons: positive; negative; zero; text. A few that come up at work:
#,##0_);(#,##0): negatives in brackets, with a space after positives so the digits line up.0.0"x": text in quotes is shown as is, so 9.6 shows as 9.6x.#,##0,"K": each comma at the end divides the display by 1,000, so 1842500 shows as 1,843K.00000: zeros pad the number, so 42 shows as 00042.ddd mmm d: Wed Mar 4.
As always, the value in the cell doesn't change; only what it shows.