i gained some experience with ARM and especially with STM32 MCU models.Now i wanted to start learning x86_64 architecture and my initial goal was going towards it. Is there any good development boards and Debugging solutions available on market. what i mean is, affordable solution which does not require billions of investments, NDAs and other corporate world stuff in order to get my dirty hands on intel CPU with a debugger.
A development platform for x86_64 is called a "personal computer" or "virtual machine"
The latter is particularly helpful if you are doing odd things at kernel level which might cause a crash, as you can run something like qemu in a mode which instruments the target VM. Otherwise you might need hardware that supports a solid low-level interface, eg, something like an actual UART to use with a tool like kgdb.
For ordinary user mode development-under-an-os of course, you can simply target the machine you posted your question from. That's even viable for kernel module development if you don't do anything too odd, though perhaps at the cost of the occasional forced reboot.
Technically chipmakers like AMD and Intel probably provide boards to industrial partners for the evaluation of brand new CPUs, but those are practically irrelevant to you, and precisely the kind of unobtainable, NDA-laden things you are trying to avoid.
Practically, get a motherboard from any of the manufacturers, (Asus, Acer, Gigabyte etc) and build a PC around it.
You may find motherboards in smaller form factors, ITX, Mini-ITX and so on that are more useful in custom systems; but probably lower performance or more limited in expansion slots.
There is no lower level "EVM" unless you go back to about the 80186 era because there really isn't a history of designing the chips themselves into embedded systems.
You may find motherboards dedicated to embedded systems e.g. with VME bus form factors, and ruggedised for industrial applications - but these will be vastly more expensive than commodity PC motherboards.
Or virtualise a sandbox on your existing PC per the other answer.