I even recently bought an old crappy S9 from eBay to play around with this idea but when i check solo.ckpool.org i saw that the reason it exists is because you cant mine to your own bitcoin core node.
Actually it's still possible. See these guide written by @nullama.
[Guide] Solo mine testnet bitcoins with cgminer, Bitcoin Core, and a Compac F
[Guide] Solo mine testnet bitcoins with bfgminer, Bitcoin Core, and a CPU/GPU
Why is this? is mining to your own node some lost art and how do these pools do it? like how else would they get the mining work anywhere else but the node? could someone please explain this to me? 
Those solo pool (such as solo.ckpool.org) are the one who give the work to your mining hardware. Obviously they run their own full node.