Which other companies ? They simply don't actually exist yet. Twitter is the only one in the X umbrella that is more than an idea of what could possibly be.Twitter is now a subdivision of a wider holding group: this holding group will be running a lot of these other operations as separate companies. Presumably these companies will have access to Twitter code (as Twitter code will presumably be the base to work from), but largely be independent. Plus, of course, presumably Musk can bolster Twitter's bottom line by having these other companies pay Twitter for code access, paid out of their independent budgets and so essentially paid for by investors.
There are only vague promises, no buisness plan or any actual investment. Musk has not made any mentions of putting more of his own money on the table and everyone else is just watching.
It is all just empty words. Until anyone actually commits a couple of billions to those plans, the other X-companies get proper names, adresses and buissness plans and hire people, i don't believe it. It is pretty much the same as Musks promises of homes on Mars. Theoretically possible but won't happen.
Also, if twitter goes down, insolvence proceedings will come for the code and data, it it was used elsewhere.