They are all held by 'Depository Trust Company' in the name of 'Cede and Company'.
https://en.wikipedia.org/wiki/Depository_Trust_Company
https://en.wikipedia.org/wiki/Cede_and_Company says
> Cede technically owns substantially all of the publicly issued stock in the United States.[2] Thus, investors do not themselves hold direct property rights in stock, but rather have contractual rights that are part of a chain of contractual rights involving Cede.[3]
Modifying the last link in that chain isn't such a big deal.
I still wonder what might happen if the broker went bust and owed X different people various fractions of a single Berkshire share.
Legal liabilities ultimately are generally settled in dollars, though if an entity “goes bust” they tend to be written off or resolved at a small fraction of their theoretical worth based on the liquidation value of the remaining assets.
Your arrangement with your broker for fractional shares is similar, though not identical. They're not just selling you a derivative product that they may optionally hedge with BRK/A.
Well, a share of stock can have two true owners, it's just that public stocks don't.
The legal ownership structure isn't hard to find out; it is actually mandated by law. All publicly traded stocks are owned by the company set up for that purpose. Doesn't matter whether you're the contractual owner of a tenth of a share or the "sole owner" of a full share; the share is owned by DTCC.
You could form a contract with your co-share owners describing how you allocate that benefit, just like would be necessary with a house with co-owners.
I have no idea if that happens, but it's certainly just as possible.
I voted for part of a politician in my electorate.
I find it much more likely that it's actually your broker who truly owns the share and you have a contract with them. This is not at all like a house which can actually have two true owners.
That doesn't mean there's anything wrong with such an arrangement, of course.