How can I download user manual for iMac 2019 21.5

Adding to rkaufmann's comments, most manuals will be tailored to the OS version you are running since that determines the computer's behavior. Without the OS, the computer is just empty hardware and you can't do anything with it until an OS is installed.

So, what OS version is yours on?

Here is another place to start - the page offers versions for 4 different OSs:

Find the manual for your Mac – Apple Support (UK)

The place to start is:

https://support.apple.com/manuals/imac_2

Is there anything specific you are looking for help on? Most computers do not come with manuals these days as they simply are not needed. I just had to buy a Windows PC (a must for my Astrophotography hobby) , of course there was no manual on the hardware of Windows.

There is no official iMac manual but in Mac of the apps windows there's a Help file. For example in the. Finder it's here:

Most even app, Apple and 3rd party, has a Help document under the Help menu item