I have Moto G gen 3 updated to latest system update, not rooted.
I need a simple solution to keep my phone's screen awake when it's plugged in.
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There's a checkbox exactly for that in the developer options, you'll just have to enable them to show up.
To do so, locate the about/version information in the Settings app and tap the build number repeatedly until you get the confirmation.
Then go back and look for the Developer Settings. Make sure to not touch any option you don't understand. Wrong values might make your device unresponsive or unusable, they could also compromise your data's security.