What is the definition of the meta keys fn,control,option,command (mac) and fn,ctrl,alt,win (windows)?

IBM defined Windows keyboards. First the IBM 84 key keyboard then the 101 key keyboard. These did not have a Windows key or a Context menu key. The keys for these functions are F10 for menu access and Shift+F10 for context menu. (Shift+F10 introduced in Office 6 as Win 3.11 had no context menus).

The release of Windows 95 and the start menu meant a key was needed for the start menu. Microsoft introduced two Windows keys and one Context menu key on their keyboards. It pretty much just did the start menu. EDIT Ctrl+Esc is the start menu access key for keyboards without a Winkey.

In the last 25 years the keys been overloaded an nearly all other keys, if not all, are used by applications.

See https://en.wikipedia.org/wiki/Computer_keyboard#Types_and_standards

That is why what key is on the keyboard.

As to why they do what they do - Common User Access (CUA) is a standard for user interfaces to operating systems and computer programs. It was developed by IBM and first published in 1987 as part of their Systems Application Architecture. Used originally in the MVS/ESA, VM/CMS, OS/400, OS/2 and Microsoft Windows operating systems, parts of the CUA standard are now implemented in programs for other operating systems, including variants of Unix. It is also used by Java AWT and Swing. See https://en.wikipedia.org/wiki/IBM_Common_User_Access#Description. I've used it in VM/CMS, OS/2, and MSDos / Windows.

See https://en.wikipedia.org/wiki/IBM_Common_User_Access#Description

As to why F4 is F4 https://devblogs.microsoft.com/oldnewthing/20180918-00/?p=99765

Why Ctrl+ScrollLock is the same as escape https://devblogs.microsoft.com/oldnewthing/20080211-00/?p=23503

Why one should not use Ctrl+Alt+ as a shortcut https://devblogs.microsoft.com/oldnewthing/20040329-00/?p=40003

Edit 2

To give you an idea of the Winkey evolution this is what it did in 2003 (when I last updated the list). Note the different teams - the hardware team (Microsoft mice and keyboards), The office team, and it appears some individual programmers from Windows 98 team.

Key Description 
Winkey or Ctrl + Esc Opens the Start Menu. 
Winkey + A Older Intellitype: Opens Accessibility. 
Winkey + B Set focus to the first icon in the System Notification Area. 
Winkey + C Office XP: Correct text in speech recognition.
    Older Intellitype: Opens Control Panel. 
Winkey + D Minimises and hides windows that can't be minimised or Unminimises/unhides all windows. 
Winkey + E Starts Explorer. 
Winkey + F Starts Find Files or Folders. 
Winkey + Ctrl + F Starts Find Computer. 
Winkey + G Cycles through Gadgets in the Vista's Sidebar. 
Winkey + H Office XP: Handwriting recognition. 
Winkey + I Older Intellitype: Opens Mouse Properties. 
Winkey + K Older Intellitype: Opens Keyboard Properties. 
Winkey + L Lock Workstation or Switch User (if Welcome Screen is on).
    Older Intellitype: Logs off Windows. 
Winkey + M Minimises all windows that can be minimised. 
Winkey + Shift + M Unminimises all windows. 
Winkey + P Older Intellitype: Opens Print Manager. 
Winkey + R Starts the Run Dialog. 
Winkey + S Older Intellitype: Toggles Caps Lock. 
Winkey + T Office XP: Toggle speech dictation mode.
    Vista: Cycles through Taskbar buttons, Press Enter or Space to 
    activate a window. This is same as Winkey + Tab in older versions. 
Winkey + U Accessibility Utility Manager (Called Ease Of Access in Vista) 
Winkey + V Office XP: Toggle listening status of the microphone.
    Older Intellitype: Opens Clipboard Viewer. 
Winkey + X Vista: Opens Mobility Centre if Windows thinks you have a 
    mobile PC. Mobility Centre is included in many Vista editions but won't 
    run unless it thinks it is a mobile PC. 
Winkey + F1 Starts Windows Help (F1 start help for the current application, if the focus is in Explorer or the desktop F1 also starts Windows Help). 
Winkey + Tab Cycles through Taskbar buttons, Press Enter or Space to activate a window.
    Vista: Cycles through open windows using Windows Flip3D. Release keys to activate window. Pressing Escape cancels switching windows. 
Winkey + Ctrl + Tab Cycles through parts of the desktop and taskbar.
    Vista: Starts Windows Flip3D. Use Arrow keys, Tab, or Shift + Tab to navigate and Enter or Space to activate. Pressing Escape cancels switching windows. 
Winkey + Break Starts System Properties. 
Winkey + Space Brings Vista's Sidebar to the top.
    Older Intellitype: List Intellitype Hotkeys. 
Winkey + Alt + Enter Starts or switches to Media Centre's Start screen. 
Winkey + <a number> Computer specific function set by the computer manufacturer. Very few computers use this.
    Vista: Activates Quicklaunch icons. Winkey + 1 is the first icon, Winkey + 2 the second, etc. 
Winkey + Up or Down Arrow Keys Windows 98 Only: Changes magnification levels in Magnifier 
Winkey + Page Up Windows 98 Only: Toggles Colour Inversion in Magnifier 
Winkey + Page Down Windows 98 Only: Toggles Mouse Tracking in Magnifier 
Winkey + Ctrl + <a number> [0-5] In Vista's Photo Gallery (but not Media Player or Explorer) rates an item as 0 to 5 stars. Winkey + 0 doesn't work on my computer. 

Here is Wikipedia's list of Winkey by version https://en.wikipedia.org/wiki/Windows_key