how to use a windows pc to download macos 10 el capitan for imac 2008 (A1224)?

Back up your data, check your applications for compatibility, click here, and then on the download link for El Capitan. Using a PC to download won't work for High Sierra or newer, and you won't be able to install the OS on the PC in any case.

(221983)

The El Capitan installer is a DMG file, which can be downloaded to the PC the same way as any other type; from there, it can be moved to the Mac through an external drive or network without any special tools.

They're only likely to have a problem doing this if the Mac doesn't have a bootable OS installed.

(222037)

A Qualifying Computer for macOS 10.11 is usually any Mac from 2008 to 2015 (there may be exceptions). You can use this article to see which versions of macOS are compatible with various Apple hardware. The host computer used to download and extract the 10.11 installer does not need to be currently running 10.11.

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

Niel wrote:

The El Capitan installer is a DMG file, which can be downloaded to the PC the same way as any other type; from there, it can be moved to the Mac through an external drive or network without any special tools.

They're only likely to have a problem doing this if the Mac doesn't have a bootable OS installed.

FYI, they still need to be using a Qualifying Mac (see my previous post for specifics) since the contents of the .dmg file is just a .pkg file which needs to be run on a Qualifying Mac capable of running macOS 10.11 in order to extract & create the macOS 10.11 installer app and place it into the Applications folder.

A Microsoft Windows computer of any kind can not perform this action - ever.

The action in this question can Only be performed on an Qualifying Apple Computer 

To add, some Users have claimed Transmac can do this which is false.

So, if you are considering that option, suggest not as it will be a waste of time and effort.