The 1st amendment in the US constitution prevents the government from stifling speech. Being banned on privately owned and operated social media is not a violation of your right to free speech.
The US Constitution only prohibits the government from inflicting cruel and unusual punishment. It says nothing about preventing a privately owned and operated company from doing so.
Yes. The constitution doesn't include Section 230 either. Other laws besides the constitution also regulate the activities of privately owned companies. This doesn't have anything to do with the government not being able to abridge the free speech of its citizens.