Quick answer: The fastest way to find the public IP address your connection presents to the internet is to open WhatIP IP Lookup. Your public address is the one external websites can see from your connection.
Public IP vs the address on your device
Your phone or computer can have a private local address while your router or carrier presents a public address to the internet. These are different layers of the same connection.
Windows, macOS, iPhone and Android
For the public address, you do not need to open a command prompt or settings screen: visit WhatIP and read the address shown at the top. If you need a local address for troubleshooting, check your device’s network settings instead.
What you can learn from the public IP
An IP can be associated with an ISP or organization, network range, approximate location and sometimes an ASN. For a deeper look, try IP Location and ASN Lookup.
See also: how NAT works and CIDR and subnets.
Want to see this in practice? Check your own connection's IP, location and ISP.
Check my IP address