The authority of the sages of Israel to demand verses after the destruction

Hello Rabbi,
I wanted to understand a little more about the authority of the Talmud and the sages in general. (Of course, I had studied Rambam in the Commentaries and the Commentaries there before.) We are bound by the words of the Great Court because "you shall not depart," and supposedly only they have the authority to demand verses and to amend regulations (especially against the Pasha).
After the destruction, the Talmudic court wrote their commentaries on the Mishnah, and we have taken upon ourselves to act in these commentaries in accordance with the words of the Great Court (to require a Great Court in wisdom and in the relevant places, etc.), and this is simple and clear.
Now, assuming that "La Tesur" refers only to the Sanhedrin, the question arises, why in the first place do the (later) Mishnah or Talmud scholars themselves have the authority to demand verses and the like against the plain text? This is only permitted for the Sanhedrin, isn't it? Similarly, why are they allowed to amend regulations themselves? (This explanation is clear. But from the verse "La Tesur" it seems that it is only for the Sanhedrin..)
It is clear that, if it is within their authority, I have the right to impose the procedures of the Great Court on them, for I have the right to accept any stricture (as long as it does not contradict the plain meaning), but it seems to me that in later regulations that removed a commandment (blow the shofar on Shabbat) or changed verses from their plain meaning (eye for eye), it is not clear why this is legitimate… (Even in Rambam, he refers to the requirement of verses only to the Great Court and relates only to it the phrase "do not deviate").
(All of this, of course, refers only to new regulations and interpretations, and not to ancient Sanhedrin traditions that they put down in writing)
thanks