why am I seeing DNS queries for other nameservers such as those owned by Google, Facebook, Cloudflare, and etc?
Why would I need to do that? The default/expected behavior of Unbound should be to directly query the root servers.
My suggestion is based on me assuming you did know how it works, and you like to alter that by controling in detail which servers to use, aka custom settings. Greelan is correct in how it does work.
I would have thought you are seeing normal behaviour for a recursive resolver. For example, if there is a DNS request for facebook.com, unbound first queries the root servers. The root servers respond with the TLD nameservers for .com. unbound then queries one of those TLD nameservers. That TLD nameserver responds with the authoritative nameservers for facebook.com. Finally unbound queries one of those authoritative nameservers, which responds with the relevant records for facebook.comLet me know if I have misunderstood your question