First, here’s the full text, adopted in 1789:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
This covers a LOT of rights we think of (and maybe even take for granted) when we think of American democracy: freedom of speech, freedom of the press, freedom of religion, separation of church and state, right of assembly/protest, and the right to demand action of your elected officials.
The first thing to know is that only the government has to comply with the constitution. It’s a contract between the government and the people.
Private companies like Twitter have their own contracts—user agreements—with users. When you sign up for the service, you agree to abide by its terms. That is why Twitter is not violating the constitution by banning Trump.
I’m not saying it’s good that Twitter has so much power over public discourse, only that the constitutional right to free speech is not part of that analysis. Private companies also do not become government actors when they go “public” on the stock market. That’s a totally different thing.
The second thing to know is that freedom of speech is not absolute. It can indeed be restricted by the government. So the government can and has enacted content-neutral restrictions on the “time, place, and manner” of certain speech thought to be especially dangerous.
The third thing to know is that statutes are subordinate to the constitution, and SCOTUS gets to decide if statutes are unconstitutional.
Take the Civil Rights Act. A good question is how the government can force private businesses to not discriminate against people on the basis of race, sex, etc. The answer is that Congress enacted a statute that governs private business: in other words, it regulated in this area.
Private businesses have challenged the Civil Rights Act itself as unconstitutional but SCOTUS said no, Congress is empowered by the commerce clause of the constitution to adopt it (the commerce clause is about free movement of goods and services through the country).
There have also been analogies of the Twitter “purge” to the “gay wedding cake” case in which a Colorado baker was allowed to refuse service to a gay couple.
The SCOTUS cake case was decided on narrow grounds, but the point is a state agency in CO said the baker couldn’t refuse to bake the cake, and he said that was a violation of his freedom of religion. SCOTUS said the state hadn’t been neutral in its application of the free exercise clause, and undid the State’s decision. So that case was not about free speech, it was about free exercise of religion.
There’s a lot more to say on all of this. But hopefully these points offer some context for recent events.