There really is no single way to do this. For your network devices, you should have an IPAM (IP Address Management) database of the assigned addresses, which should be statically assigned for network devices. Some network devices will have multiple addresses. For example, routers need an assigned address for each interface, including virtual/logical interfaces, e.g loopbacks. If you do not have such a listing, you need to connect to each network device to discover the addresses assigned in the devices.
For devices using DHCP, you should be able to get that information from your DHCP server assignments.
For devices with statically configured addresses, you will have more trouble. You can get into each device to see how the addressing is configured. You could look in the ARP table of the router for each device to see the IP/MAC address resolutions, but remember that ARP table entries time out, and the ARP table may be incomplete.
Unfortunately, if you did not keep up with this from the start, this will probably not be easy to create from scratch.