I am looking into setting up SFDX and one of the first things is to enable Dev Hub in the production org. We are external consultants and I want to verify the impact this will have on the org. The customer is still using classic screens and I do not want to affect them. I am also unfamiliar with the concepts regarding 2nd generation packages. Does anyone have a recommendation for the best place to read about it?
Turning on Dev Hub doesn't have any other effects on Production, saving the most edge of edge cases (e.g., if something reacted poorly to the mere presence of the objects that track scratch orgs that are created). However, you can't turn it back off.
Enabling Dev Hub in a production or business org is completely safe and won’t cause any performance or customer issues. Dev Hub comprises objects with permissions that allow admins to control the level of access available to a user and an org.
Users won't see those objects unless they have permission to, so the only negative outcome I can even speculate about would be an integration running on a user account that does have permission to see scratch org records and which has some undesired behavior when it sees a new kind of sObject.
There is a Trailhead module providing an introduction to unlocked packages.