Crossfire was a feature supported by AMD for developers to implement in games for dual or triple, or quad GPU support.
AMD stopped supporting Crossfire with the arrival of the Radeon RX 5000 series GPUs.
They removed the bridge adapter way back during the VEGA days, supporting multi-GPU support over PCI-E. Then later took away the option to enable Crossfire in Radeon Settings when it come to the RX 5000 GPUs.
MGPU is a feature of DirectX 12 that Microsoft created that provide more multi-GPU capabilities than AMD's Crossfire or NVIDIA's SLI.
You can have mismatched GPUs with DX12's MGPU processing the same game together, you can even mix AMD and NVIDIA GPU's in MGPU.
The then thing is though very few game developers have implemented full support of MGPU which is why multi-GPU setups are largely thing of the past.
The only game I know that fully supports MGPU is Ashes of the Singularity. Shadow of the Tomb Raider I believe also had some MGPU support, but a vast majority of games don't, and running multi-GPUs together on games that don't support MGPU (or older Crossfire/SLI) will usually result in less fps performance than if you just ran a single GPU.
I missed the writing on the wall in 2019, but I read the tea leaves of every Tech Tuber crying about dual GPU being dead, and I as result sold my dual VEGA 56 GPUs last October and prepped and eventually got a 6800 XT to replace it.
I advise not to waste your time on MGPU / Crossfire/SLI etc. It's a dead technology thanks to lack of development support.
