Ok, I apologize if this info has been covered in other posts, but just a quick summary, as it addresses some questions regarding what Bulova actually manufactured.
Bulova owned the factory(s) in Switzerland that manufactured ebauches for their watches. An ebauche is a raw movt, and can be modified to a particular finished design. There are references to the factory being operational by 1912. Before US modifications to raw ebauches became common practise for Bulova, I suspect that complete movements and watches were manufactured here in Switzerland.
The US factory made plates, dials, and escapements that were fitted to these, and other ebauches, which Bulova purchased from various leading ebauche manufactures, like ETA, FHF, Felsa, A.Schild, Aurore-Villeret, Sonceboz, Glycine, Peseux, Unitas, FEF (Fleurier), Le Phare, Junghans (alarm), Gruen, Venus, Michel, Liengme, Recta, and possibly others. The ebauches from Gruen were of the famed Aegler company, jointly owned by Rolex, Gruen and Aegler.
In 1930, Bulova was manufacturing components, that is, plates, dials and escapements in the US at their factory. By about 1940, Bulova was manufacturing complete watches, and by the early 1950s, they were producing over a million watches per year, approximately half being imported Swiss, and half Domestic (US). It seems the watches were always cased and assembled in the US. The cases, as we know, were sourced from various casemakers.
So Bulova did indeed use US made in house movts as well as Swiss in house movts, but these are in the minority. Keep in mind, a lot of major compnanies like Rolex, Breitling, and Panerai only started producing their own in house calibres relatively recently. Panerai, for example, once famously used a Rolex movt in their watches, but it was sourced by Rolex from Cortebert.
As a watchmaker, I find this to be very defining information, and I have a deep respect for Mr Joseph Bulova, and what he did, especially when he re-trained war vets to become watchmakers at his school, many of whom were physically handicapped or had other trauma related to the war, and would have probably been marginalized and under employed, if at all. My hat is off.