The constitution applies to the government. It establishes the government and defines -- and therefor limits -- what the government is allowed to do.
Corporations are considered "legal persons" for the purpose of applying the law to them in a convenient and organized way, but in real life, corporations are just organizational models employed by human beings for the purpose of coordinating their activities.
The restrictions applicable to what the government is allowed to do to "people" as defined in the constitution apply regardless of what organizational models those people are using to coordinate their activities. Ultimately, everything in society reduces to people, and the government is not entitled to use reified abstractions to escape the constraints on its authority.
Corporations have First Amendment rights as ruled by Citizens United v. FEC. Even though corporations don’t have a vote (which is its own can of worms because of their economic power, money = vote), they still enjoy some of the same constitutional protections as individuals do.
No, there's no such reasoning in that decision, which confirmed that speech itself is protected by the first amendment, regardless of who originates it.
And this ruling had little to do with any of that -- the first amendment challenge was that the ban imposed content-based burdens on the speech of the users of TikTok, and the court ruled that it did not. So the ban therefore survived the challenge under intermediate scrutiny.
The domestic vs. foreign ownership element of the ruling only pertained to the evaluation of whether there was a compelling government interest in enacting the ban, not whether the government was exempt from first amendment scrutiny at all.
Forced corporate divestiture is a thing, for example Merck.