Okay, so because of comments, I looked this up.
This beaver is a rehab animal. He was found on the side of the road when he was no bigger than a potato. He is still young in this video (you do not realize how big adult beavers are, trust me) and is not ready to leave his home or family. Yes, beavers are social and they live in families. For him, his rescuer is his family. But he will be ready to leave at the age of two or three, when a wild beaver would leave his parents and seek out a territory of his own.
Anyway, he gets plenty of outdoor time and is also being taught to swim and do normal beaver things. He WILL eventually be released. So him building a dam out of plushies is not as sad as it might seem.
The truth is, animals are typically happy if they get to exercise their instincts, even if the environment is an artificial one. But in general, a wild animal should be either in the wild or in a captive setting that replicates its natural environment. Beavers are NOT good pets. Fortunately, this guy is not being kept as a pet. So it’s okay. Enjoy the video, and check out Raising The Wild on YouTube if you want updates.