Dell Windows 10 OEM

You are correct. The product key is essentially attached to the board, so if you restore your laptop to factory, it will be automatically activated. This is why Dell does not attach the product key to the case on a sticker anymore.

However, just be aware that if you were to reinstall a new retail copy of Windows 10 (Outside of the Dell restore process), the embedded key would not work in that situation.

But yes, that hardware key will still be there is you update the BIOS or factory reset the computer using the Dell restore procedure.

Yeah I have reimaged many Dell computers with own SOE, and they have activated nicely on our domain.

But when I take computer off domain and reconnect with different domain. It's not activating

Is there any way to activate manually using any command line?

We only using oem licencing not volume licencing

OEM Windows 10 Licensing is held in the Windows Licensing center. That is how it checks the licensing on a regular basis.

But when I take computer off domain and reconnect with different domain. It's not activating

and you say " I have reimaged many Dell computers with own SOE"

That is probably the issue. You cannot image with OEM, only with Volume Licensing and Sysprep.

actually oem licensing uses a different process than domain licensing.  domain licensing uses volume licensing and a KMS (key management server) . to utilize the oem licensing you have to re-use the factory restore process from the recovery partition.  chances are when you took it off domain and reimaged you were utilizing the other locations MDT (microsoft deployment tools) installation.  you need to make sure you are recovering from the recovery partition if you want to use the built in license.

We use OEM licensed Windows 10 Computers (no image copying) on domains without any difficulty or any loss in activation at any time

and you can easily use OEM licenses on a domain.  however if you want to continue to use OEM licenses then you have to be sure to use the recovery partition to restore.  if you use imaging from the domain then you will be using volume licensing and KMS.  the two technologies can happily coexist

then you have to be sure to use the recovery partition to restore.   Or a backup of the OEM image to be used on the same machine.

Yes of course, the licensing must be held to the machine.

"Dell precision 5530 with Windows 10 OEM. How the activation works?"
The Windows 10 installer is set to look for an embedded key in preference to any other activation method. If it's not found and there is no digital entitlement it requests a retail activation key.  Volume licences use a separate installer that ignores the "check for an embedded key" process.
One consequence of this can be you try to install Win 10 Pro on an OEM machine and the installer reads the embedded key and installs Home because that is what the embedded key is valid for.

"Is windows 10 keys are embedded in BIOS?"
Yes for OEM

"Will it still be there if I upgrade BIOS and factory reset it? "
Yes - the embedded key can be edited or deleted but this is absolutely not recommended.