They typically work during the late hours of the night.
https://www.vergemagazine.com/articles/beyond-the-guidebook/397-the-other-nightlife-in-buenos-aires-the-story-of-argentinas-cartoneros.html
I've conducted a search but couldn't find recent recommendations. We're interested in 3-4 hour walking tours covering various parts of the city, focusing on architecture and history. I explored Tours By Locals, but many of them were priced over US$500. While I understand they might offer...