Inside the Ribbon Shortcuts in Excel

To do this action	Press in Windows	Press in Mac
Activate Selected Buttons	Space	Space
Expand / Collapse the Ribbon	Ctrl+F1	⌘ +⌥ +R
Move to Left Command	Shift+Tab	⇧ +Tab
Move to Right Command	Tab	Tab
Move to Sub-menu, when a Main-menu is Open	←	←
Move to the Next Command in Open Menu	↓	↓
Open a Context Menu	Shift+F10	Fn+⇧ +F10
Open the List for Selected Command	↓	↓
Open the Menu for Selected Button	Alt+↓	⌥ +↓
Select Active Tab and Activate Access Keys	Alt
Select Active Tab and Activate Access Keys	F10

For More Shortcuts (Click on the links below):

Excel Ribbon Tab Shortcuts for Windows

Shortcuts to Make Selections in Excel

Formatting Cell Shortcuts in Excel

Frequently Used Keyboard Shortcuts For Mac & Windows in Excel

Excel Cell Navigation Shortcuts For Mac & Windows

Formula Shortcuts For Mac & Windows