In my post I clearly stated I have Windows 10 ISO. Why do they refer my thread to one that is looking for Windows ISO? I already have it.
You mention the fact you have a Windows 10 ISO but it's to large to write to a DVD. I took this as an indication you were not aware of how to properly create a Windows 10 installation media.
I'm looking for boot image FOR Windows, not boot image OF Windows. Why does everybody think I want to burn Windows 10 ISO? I want to archive files while making the DVD bootable.
You indicated you were looking to create a bootable image. You specifically talk about the Windows 10 ISO several times in your question. When I flagged your question as a duplicate, at the time, you had not submitted any comments under the only answer you received. The answer you receive was nearly a duplicate of an answer to that of the duplicate question.
- What I have is the Windows 10 ISO which is more than 5G and cannot fit on a single layer DVD.
- I have Windows 10 1909 64 bit ISO whose size is 5,294,394 Kb, obviously too big for a single layer DVD.
Your current question does not indicate what bootable environment you are expecting to use. So it's unclear if you are looking to use WinRE which makes the most sense, but isn't exactly the simplest to create, without any knowledge of how to create a custom WinRE image.
You only later qualified what you were actually looking for, but even your clarification, is not all that detailed.
I want to archive files and, at the same time, make the DVD bootable.
In order to answer this question, we need to know what bootable environment you want to use, your question only mentions the Windows 10 installation media.
I need a boot image to burn a bootable DVD for Windows 10.
(Is there) anyplace to download a boot image or anyway to extract the boot image from an ISO file on hard disk?
This specific question is outside the scope of Super User. How is anyone suppose to provide a bootable ISO if we know nothing about what you are trying to achieve. Your question in it's current revision makes no mention of the fact you want to backup up your files and have access to those files in a bootable environment.
So while your question might not be a duplicate of Where can I get a clean ISO of a specific build of Windows 10?, it's currently not clear enough, to actually answer your question.