"No amendment to the Constitution is absolute. You can't yell 'fire' in a crowded movie theater and call it freedom of speech. From the beginning, you couldn't own any weapon you wanted to own," he said, a line used during his 2020 campaign.
Neither of these claims is really correct. The claim that yelling fire in a crowded theater is not free speech comes from a court case in which someone was being prosecuted for handing out fliers criticizing the draft; it was later overturned, is widely recognized as a badly decided case, and is wrong. There are actions where it is recognized that freedom of speech does not necessarily excuse them despite the fact that they are done with speech -- things like defamation, obscenity, incitement to violence -- but precisely because of freedom of speech, the handling of them is very narrow and precise (and notably all such cases pre-exist the Constitution, so having narrowly defined laws against them does not restrict or infringe the freedom of speech people have always understood themselves to have). In any case, shouting 'fire' in a crowded theater is not one of these acts that use speech but are not wholly excused by freedom of speech.
And it is in fact the case that in the United States you can, in principle, own any weapon you want to own. It doesn't even make sense to say otherwise. In 'the beginning', the most important example of naval power -- warships being the most powerful weapons in the world at that time -- could be privately owned. The US Constitution literally has a clause assuming that Congress will issue letters of marque and reprisal, which are government authorizations for private ships to engage in acts of war. If you, as a private American citizen, want to own a tank or a fighter jet today, you are entirely within your rights to do so. (We don't have tanks and fighter jets everywhere because they are prohibitively expensive to maintain, and moving them around is immensely difficult. It's the same reason why, despite the fact that it is legal to own cannons, we have never had large numbers of cannon owners. Nothing prevents any of us from owning an aircraft carrier except that none of us could afford it. But a few tank enthusiasts, for instance, own their own fully functioning tanks and spend a large portion of their income every year maintaining them, just for the entirely legal fun of it.) Some things are very highly regulated, to be sure, but if you're willing to take the years to jump through the legal hoops, the only weapons you can't in principle own are cases where the impediment is incidental to owning the weapon itself -- like the patents being held by the military, or your attempting to store the weapon too closely to a population center, or not getting proper permitting and licensing for an alteration, or you are attempting to buy a weapon can't practically be made or sold for some reason.
You can decry this, if you like, as unfortunate, but such is the American way. For all practical purposes, you can in fact say what you want to say and own the weapons you want to own if you are an American citizen, without the government prohibiting it, because the Constitution gives you that right.
In an absolute sense, no Constitution is absolute -- the fact that we can get rid of them or amend them is an obvious proof of that -- but, again, for practical purposes, Constitutional amendments are in fact absolute, in the sense that they are not supposed to be qualified or restricted except by Constitutional amendment. That's the whole point of a written constitution. Both federal and state governments are always trying to give themselves wiggle room in the interpretations, and perhaps sometimes this succeeds, but the whole point of a written constitution as a safeguard is that neither legislature nor executive are supposed to be able to impose exceptions or qualifications on their own authority. Now, if President Biden wants to argue that there should be a new Constitutional amendment, he is entirely within his rights as an American citizen. But, unless we've just decided that Presidents should be able to do whatever they want, it takes a Constitutional amendment to restrict a Constitutional freedom.