Built-in shortcut list. Cmd+/ opens Google Sheets' own keyboard shortcut panel. Searchable and organized by category, particularly useful for finding formula shortcuts or less-common formatting combinations without leaving the spreadsheet.
Data range navigation. Cmd+Arrow moves the cursor to the last filled cell in the arrow's direction within a data range. In a large spreadsheet, Cmd+Down jumps from the current cell to the bottom of a contiguous column of data in one keystroke. This is the same behavior as Excel on Mac (also Cmd+Arrow) and one of the highest-value shortcuts for anyone working with real data.
Selecting data ranges. Cmd+Shift+Arrow extends the selection to the edge of the current data range. Combined with Cmd+Shift+End to select to the last cell with data, these are the selection shortcuts that make large-range operations practical without a mouse.
Formula entry on Mac. Entering edit mode in a cell, navigating within the formula bar, and confirming or canceling a formula entry all have Command-based shortcuts on Mac. The exact shortcuts are in the table.
Paste values only. Pasting without carrying over source formatting or formulas has a dedicated shortcut, useful when you want the numbers but not the formatting from a copied range. Covered in the table above.
Browser shortcut conflicts. A small number of Command combinations may be captured by the browser before Sheets sees them. The table above notes the most relevant cases.