With nuclear matters like this, it's important to keep in mind that nukes come in basically two pieces: the warhead (the "nuke" per se), and the delivery vehicle (the rocket, the "reentry vehicles," etc.).

Sarmat is a rocket, a delivery vehicle. They have not disclosed what kind of warhead is in it. It is unlikely to be something completely new, because they are not testing nuclear weapons anymore, and when you can't test what you make you tend to get conservative. So the odds are that they will just keep another warhead in there.

If they were retiring a warhead variety, that involves removing the fissile material (the plutonium and/or uranium), and usually re-cycling that one way or another. Sometimes it is just stored, sometimes it is formed into new shapes and used in other weapons, sometimes it is blended down and put into reactors. The US and Russia had a big program in the post-Cold War called "Megatons to Megawatts" that involved the US buying blended-down Russian warheads and using them in its power reactors. In the US, warhead dismantlement takes place in fairly isolated areas under huge piles of gravel ("Gravel Gerties") so that if there is an inadvertent explosion (not a nuclear weapon, but something that would spread plutonium around), it will just kill the person dismantling the warhead and not people down-wind.

If missiles are being retired, those can be dismantled and destroyed like any old missiles. Again, the warheads can be removed and kept. Both the US and Russia have large stockpiles of "un-mated" (not attached to a delivery vehicle) warheads that, by themselves, are useless, but can serve both as a "hedge" for future use, or as bartering chips in future negotiations.

(Separately, the Sarmat cannot destroy France or Texas, I mean, not anymore than any other existing MIRVed ICBM can. The amount of energy to "destroy an area the size of Texas" is ridiculously high, far larger than any warhead ever developed, and any warhead of that size would weigh about the mass of a Space Shuttle, and as such not be a very useful weapon. Think on the order of thousands of megatons. The trend in warheads since the 1970s has been in the range of 500 kt or so, because that can be made into a small-enough size that you can put many of them on top of one missile very easily. The Sarmat sounds like another missile of this type. There is a lot of misinformation circulating about this, and also a lot of apparent ignorance in the fact that both the US and Russia have already had plenty of ICBMs and SLBMs for quite some time now.)