Questions about ASUS TUF Gaming B850-Plus WiFi

Yes, this motherboard is compatible with a PCIe 5.0 SSD. It has an M.2 slot (M.2_1) that supports PCIe 5.0 x4, connected directly to the processor for optimum performance. To take full advantage of PCIe 5.0 speeds, you need to pair this motherboard with an AMD Ryzen 9000 or 7000 series processor, as these offer native PCIe 5.0 support. Ryzen 8000 Series processors only support PCIe 4.0 on this motherboard.

No. The Ryzen 3900X is a socket AM4 CPU. The motherboard here is from the next socket generation (AM5) and there is no backwards compatibility in this regard.