The argument from TikTok is:

>Petitioners argue that such a ban will burden various First Amendment activities, including content moderation, content generation, access to a distinct medium for expression, association with another speaker or preferred editor, and receipt of information and ideas.

Sotomayor expands on this in her concurrence:

>TikTok engages in expressive activity by “compiling and curating” material on its platform. Laws that “impose a disproportionate burden” upon those engaged in expressive activity are subject to heightened scrutiny under the First Amendment. The challenged Act plainly imposes such a burden: It bars any entity from distributing TikTok’s speech in the United States, unless TikTok undergoes a qualified divestiture. The Act, moreover, effectively prohibits TikTok from collaborating with certain entities regarding its “content recommendation algorithm” even following a qualified divestiture. And the Act implicates content creators’ “right to associate” with their preferred publisher “for the purpose of speaking.”

The Supreme Court can only rule on cases brought to it. And in those cases, they are ruling on specific points of law which one party believes that a lower court misapplied. In this case, the parties asked the Court specifically to review whether a TikTok forced divestiture (not a ban, a forced sale) violated the First Amendment.

>What does this have to do with the First Amendment?

Because obviously changing the owner-editor of a media outlet has everything to do with their editorial policy. The SCOTUS just said that censorship is ok (and forcing the change of the editor is censorship, there is no doubt about it), as long as it's against another state's editorial preferences potentially having a significant audience in the country.