You can plug PCIe 3.0 devices into PCIe 4.0 ports without any problems, they are physically identical and the BIOS recognises the version automatically. Of course, you only get the bandwidth of the weakest link.
For example, I have a Ryzen 2600 in the board (for now), so everything runs on PCIe 3.0. But I have already installed a Corsair MP600 M.2 NVME SSD with PCIe 4.0, which only runs with PCIe 3.0. With an upgrade to, for example, the Ryzen 3900X, which supports PCIe 4.0, I can then also operate the said SSD with the full bandwidth of PCIe 4.0.

To your questions:
- You can run a PCIe 3.0 SSD without any problems, regardless of whether the CPU supports PCIe 4.0 or not, regardless of whether the slots are PCIe 3.0 or 4.0.

- I'm not sure if all PCIe/M.2 slots on this mainboard are PCIe 4.0 or if some are only connected with PCIe 3.0, but it doesn't matter in terms of compatibility. (It does in terms of bandwidth, of course)

Has all PCI connections, can also be seen in the description of the product😉.