How to Get Someones Internal IP on Your Network and Off Your Network

I was wondering how you could get someones internal ip address both from someone on your network and from someone you don't even know. I run Kali Linux and am trying to catch myself back up.

-Also how would I find someone on my network if theres two routers.

For example: Your main Internet and Wifi come from a NetGear Router yatayata however that NetGear Router goes through a "Nighthawk" Router to dramatically increase your download and upload speed.

(also take in note theres one or two hidden ssid for the wifi login. there is also one that isn't hidden.)

How would I know whos all connected to my network?