As a non-American, my question is why does WeChat have first amendment rights? It's not an American company.

In that framework, the US government is not to violate anyone's freedom of speech anywhere in the world. That doesn't seem to be what's at issue in this case though; the case was brought by a group of US-based WeChat users on grounds that the ban would restrict their freedom to communicate with people in China who are unable to use other communication services.

> Yes.

We might also observe that the only mention of "people" in the first amendment gives them the rights to assemble and to petition the government.

Freedoms of speech and the press are phrased as restrictions on Congress, not as rights given to anyone (and this is probably the main justification for extending first amendment rights to foreigners, which appears to be a popular but not universal viewpoint).

of Free Speech is arbitrarily granted, then it's not free speech, and that violates Prior Restraint, at least, if the Courts are not also arbitrarily deciding who's free speech deserves to be protected.

Note the subject.

So yes, the First Amendment is okay with any censoring of information by a foreign government or company.

"Article 19.

Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers."

I don't see any mention of government there. Why limit the interpretation of free speech to the US constitution? How do you reconcile ideologically or philosophically that it is not ok for US Congress to limit speech but ok for other powerful entities?

Because that's literally what the grandparent asked. You appear to have a completely different question.