It’s a bit of a “how long is a piece of string” question. As most of the volume of most brands (or at least the big name brands) is made in the same handful of factories. They might (maybe) have their own facilities for their top end carbon wunderbikes. But their volume stuff they will almost entirely buy in complete.
I know of a few brands where their bikes won’t ever actually be touched or seen by an employee of the company, or may never even be in their own premises. Or they may only briefly see the box as it arrives or leaves a distribution centre. Gets built by a third party, packed and shipped by another, distributed by another and then sold by the shop.
I wouldn’t be surprised if some of the volume factories are producing twice what giant do. Or more. But making them for half a dozen different brands.