The Scan app is where you find other player devices. Upon launching Scan it will immediately begin searching for random players, allowing you to refresh until you find a Firewall level you deem suitable to bypass. The device's that are found are devices of players who are in range of your level. For example, If you're a level 5 you are more likely to find players ranging from Level 3 - 7. A player's level is not to be mistaken by their Firewall level that shows up in the scan. You won't be able to find out the actual player's level until you have successfully bypassed their device.
Pinging a device allows you to search for any device by their IP. This could be a rogue IP you found in a Log or any other IP you know of that you're curious about.
Firewall levels lower than your Bypasser will be faster to hack and have a greater chance of success. Though a lower Firewall could also mean a less rewarding Hack as you may not get as much out of it. If you're willing to wait out the longer hacks and chance failing, higher level Firewalls could mean a better return.