I am trying to build something similar but I don't know how to get a non-functioning physical prototype built initially. What's the simplest way to get these type of aluminum cases, bands, etc?
Short of paying many thousands of dollars in prototype engineering fees, you either buy an existing device and strip out the guts, or one of these for $20 https://serpac.com/bw-series.aspx
How about buy a cheap Chinese smartwatch and reuse the case? If that works for you, it is very likely you could discover who manufactures it, and work with them to build yours.