How to view devices using my IP address or iCloud account

As far as your iCloud account, Phil0124 has pretty much covered it. However, I would add that any devices your find there will ones that have logged in with your Apple ID, not necessarily ones that are currently logged in.

As far as IP addresses, that would depend on a number of things. If we are talking about your local network, your router would control providing IP addresses to each device connected to that network ... and depending on the make & model of that router, whether or not, you would be able to find out the devices that are currently (or previously) connected to it.

Login in to https://appleid.apple.com A list of devices logged in to your Apple ID will appear there.

Your IP address is unique and cannot be used by more than one device.

If another device tries to use your IP address, you would get an error stating as much. Also the secondary device using your IP address would need to be connected to the same wifi network, as otherwise, duplication of IP addresses is impossible.