Verification code sent to device it is used for.

When 2FA id is enabled in Mac system preferences - iCloud and you turn off the trusted devices or if device is damaged, the Mac will itself act as a trusted device and a pop up window will appear on your desktop showing a map click on allow ( also in system preferences - iCloud - find my Mac box should be checked , in security and privacy location services , maps should be checked ).

After a while a pop up window will appear showing your Apple ID is being used to sign in to a new device , enter this verification code on your device to sign in a six digit code will be written , write down this code in a safe place and click on done , then enter this code in the field that appeared on the desktop , you will enter the 2FA account .

See this article also Two-factor authentication for Apple ID - Apple Support

Mr. Unko wrote:

When I sign into an Apple website the requires my Apple ID it surely does sent the required verification code needed to verify the action of signing in.

Unfortunately, this code isn't just sent to other iDevices of mine, but also the device that requests the code, making the whole process completely meaningless as it only adds to the steps necessary to sign in, but without increasing the security the slightest, which I actually consider a serious security flaw that I ca not accept.

It is not a flaw. It is working as designed. 2FA is not intended to protect the device. It's intended to protect your Apple ID. Your device should be protected by a lock code. If the device is locked, no one will be able to access it or the code.

Mr. Unko wrote:

So it is supposed to do this?

Yup, that's what I said.

You should also consider adding a landline or other mobile number as a trusted number.