There are "free" walking tours in every major city of the world. I have taken them in three cities so far, and every one was terrific. The guides were typically well educated and knowledgeable locals with other professional day jobs (professors, lawyers, musicians, etc.). Most tours were conducted daily, without any need to book or register. Just show up on time at a chosen location. Their tours were about pride in their history, culture, and cuisine. And they were not shy about sharing their opinions, on politics, economy, sports, scams, etc. They would not take you to places where they would get commissions, like some professional guides would. When asked about places to visit/shop/dine, they would offer their own favorites, not those found in the tourist guidebooks. They were like welcoming friends. Not being paid by their employers, their earning came from tips by the tour takers. A very generous tip was only a fraction of what I would have to pay for a "professional" guide.
To find them, simply Google "[City] free tour". Then go to TA for their reviews.