Chalalán is the reason people know about Madidi. It belongs to the Quechua-Tacana community of San José de Uchupiamonas, who built and run it themselves rather than hosting somebody else's operation, and it sits on a lake five hours by boat up the Beni and the Tuichi from Rurrenabaque — far enough in that the forest around it has never been logged.
Madidi runs from 200 metres to 6,000 and is credibly one of the most biodiverse protected areas on earth. What that means in practice is birds: a species list long enough that people come for that alone, plus macaws, monkeys, peccary and the reasonable chance of something larger.
The booking office is on Comercio, half a block off the main plaza in Rurrenabaque. They answer WhatsApp and say so in plain words.