Windows (File Explorer): Ctrl + A
macOS (Finder): Command (⌘) + A
Web browsers (page content): Ctrl + A (Windows/Linux) or Command (⌘) + A (macOS)
Google Docs: Ctrl + A (Windows/Linux) or Command (⌘) + A (macOS)
Microsoft Word: Ctrl + A (Windows) or Command (⌘) + A (macOS)
Microsoft Excel: Ctrl + A (Windows) or Command (⌘) + A (macOS)
PowerPoint: Ctrl + A (Windows) or Command (⌘) + A (macOS)
Command Prompt: Ctrl + A
Windows (Desktop icons): Ctrl + A
macOS (Desktop icons): Command (⌘) + A