First off, an IP-address wouldn't tell you what type of device is connected.

Your router generates a local network with non public facing IP-addresses. You cannot use that address to connect to a device from outside the network. You'd instead connect to the router that's using the public facing IP-address and then forwards your traffic to the correct device.

The router will assign different MAC-addresses (basically the network adapters serial number) their own IP-lease. If it's a dynamic lease it can change vs. static lease that means a determined MAC-address will always get the same IP-address.

Changing users doesn't change MAC-address so this means that all users on the computer will indeed have the same IP-address. However, if the computer is left off so that the time of the lease has ended you may be assigned a new IP-address.

So in short: You cannot determine what user has been online unless you've setup MAC-spoofing for individual users. MAC-spoofing is the act of manually changing the MAC-address.

You can if you have access to the DHCP server (in the example it'd be the local router) determine what MAC-address has been assigned a certain IP-address by looking at the lease itself or logs.

While you could perhaps figure out what brand of computer by looking up the MAC-address online it'd not help if someone has spoofed the MAC-address.

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In essence:

A DHCP server (router in this case) will assign an IP-address to a MAC-address. To know what device has an IP-address you'd need access to the DHCP-servers logs.

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As for comparing IP-addresses it's basically impossible to give you further information without knowing what you're actually trying to do short of telling you to read up on how IP-addresses work. https://en.wikipedia.org/wiki/IP_address