Questions about AMD Ryzen 5 5500

YES, there is.

Graphics cards are downward compatible so that one with PCIe4.0 also runs with PCIe3.0.

The Ryzen 5 5500 with socket AM4 should fit on the Asrock B550 Steel Legend according to the specifications, the motherboard is compatible with the socket. You may have to update the bios, I don't know if the version used on the motherboard already supports 5th Gen Ryzen.

Yes, there is a cooler included, the weight of the item is 447g, you can only manage that if a heat sink is also included.