Hi, Brian. I’m glad you’re enjoying the lectures.
The discipline of history, because of its reliance on written testimony, isn’t really equipped to answer your questions. They fall more under the purview of archeology.
There are multiple theories as to how, when, and why people first came to the Americas. The one that seems to be most in favor right now is that they crossed a land bridge from Russia to Alaska in pursuit of big game during an Ice Age some 12,000 years ago. But any explanation involves a decent dose of speculation.