Everything I command you that you shall be careful to do it. You shall neither add to it, nor subtract from it.
If there will arise among you a prophet, or a dreamer of a dream, and he gives you a sign or a wonder,
and the sign or the wonder of which he spoke to you happens, [and he] says, "Let us go after other gods which you have not known, and let us worship them,"
you shall not heed the words of that prophet, or that dreamer of a dream; for the Lord, your God, is testing you, to know whether you really love the Lord, your God, with all your heart and with all your soul.
If you read the Rambam (Yisodei HaTorah chapter 9) you understand why there is a verse 1 (that nothing can change), but then the subsequent verses call out idolatry specifically. Idolatry is non-negotiable. If a "prophet" calls for it even as a one-off, he is false (although his "sign" - his prediction - may well have come true and G-d made it happen). But when it comes to anything else, a temporary change may in fact be required, but it cannot be a permanent addition or subtraction.
In the beginning of Chapter 9 the Rambam brings 4 verses which support the idea that nothing can be added or removed by a subsequent prophet. However, those are best understood with the philosophical underpinning described in the preceding chapter. There the point is that a prophet is not accepted based on the strength of their miracles. Rather, due to the mass revelation at Sinai, we see that Moshe is a true prophet and thus any subsequent prophets have to be consistent with his criteria.
The Rambam then brings 4 verses. These verses (as I understand the point of using so many) is to specifically address different types of claims about why, given the philosophical presentation in the previous chapter, some prophet trying to contradict what Moshe established should still be listened to.
The first claim is "The Torah itself allows for additions or subtractions at a subsequent time, so it is fully consistent to have them." Much like we in fact say the location of the Beis HaMikdash was revealed later. This the first verse shows is not the case "Everything I command you that you shall be careful to do it. You shall neither add to it, nor subtract from it."
The second claim is that the Torah itself (including the above verse) has a built in expiration. It only applies for some period of time and then stops being applicable, leaving space for someone else to come along. For that he quotes the next verse (Devarim 29:28) "The hidden things belong to the Lord, our God, but the revealed things apply to us and to our children forever: that we must fulfill all the words of this Torah." The emphasis on the "forever."
The third claim is that although the Torah in general has no expiration date, this or that certain Mitzvah does and is given one by the Torah. For that he quotes "[This is] an eternal statute for all your generations" (Vayikra 3:17 and a bunch (7?) of other places).
The fourth claim is that although the Torah is eternal, a Prophet can come along and tell us how to interpret it, and say "this Mitzvah is not literal" or "this accepted interpretation is not valid", for that he quotes the final verse (Devarim 30:12) "It is not in heaven."
So, to pick on an example that came up on this site before, if a Muslim comes along and says that Moshe never commanded to not eat camel - first we have the philosophical point that we know that Moshe said it directly.
If he then says, but Mohammed can come along and change that, we say "nothing can be added or subtracted."
If he says "The Torah was only supposed to last until Mohammed came along" then we say "the revealed things apply to us and to our children forever."
If he says "The commandment to not eat camel was only until Mohammed" we say every Mitvah is eternal.
And if he says "Well, you have the text all wrong" or that doesn't mean camel it is a metaphor, it means a specific kind of other animal, or if the camel ate something specifically wrong, we say "it is not in heaven" a prophet can't come along and say that G-d said we got it wrong.