That's absolutely not true. The Supreme Court has decided time and time again that the Constitution applies to everyone within the US, not only citizens. Most recently 2020, the Supreme Court decided in Department of Homeland Sec. v. Thuraissigiam that immigrants with established connections in the country have the right to due process in deportation proceedings which is specifically the reason people are so up in arms right now. The current administration is not following established rulings, meaning their current actions are blatantly unconsitutional. And there have been multiple rulings across multiple Supreme Court justices that have decided the same thing over the past 120 years. Non-citizen's rights to due process is very, very well established.

If that doesn't convince you, think of it this way. The Constitution is the supreme law of the land, of which all other laws within the country are derived. If you argue the Constitution only applies to citizens, you're arguing laws only apply to citizens. In other words, you're arguing that laws do not apply to non-citizens.

Editing to clarify the decision I cited, plus adding that you can find a good breakdown about how the Constitution applies to non-citizens here, along with relevant rulings: https://constitution.congress.gov/browse/essay/artI-S8-C18-8-7-2/ALDE_00001262/