The rules of derashah are used in two ways:
The "real" way, derashah, does produce deOraisos -- actual Torah law.
Even if one follows the Rambam, Mamrim 2:1, that derashos could be invented/discovered -- let's just say "utilized" -- for the first time centuries after Sinai. And that another court may choose not to use it. One example, not given by the Rambam, is that according to Ruth Rabba, the derashah that only a male Moabite convert is restricted from marrying into the Jewish people, but a woman may, was first used by Boaz. Which then makes the closer relative's reluctance to marry her very understandable. Maybe Boaz erred?
This is not the usual understanding. Typically we follow the majority of rishonim, that while all of the deOraisos were given in Sinai, some were recorded in the text, some were hinted in the text via derashah, and some are halakhos leMoshe miSinai [laws given to Moses from Sinai]. (Understood to mean that either are not in the text at all or we forgot how they connect to their source verse, but every deOraisa does connect somehow. The latter opinion being that of the Malbim, intro. to Vayiqra.)
Thus either every derashah was given at Sinai alongside the laws, or they are rhetorical devices utilized later to support laws that were given at Sinai and thus already known. For example, to help decide in a dispute among valid interpretations of what was given in Sinai, which one should become law.
Then there are also asmachtos, connections, where the rabbis find a connection between a rabbinic law and the text of the chumash. According to the typical explanation (eg the Rambam's introduction to Mishnah, the Kuzari 3:73), this is done for mnemonic reasons, or to impress on the masses the import of the law.
According to the other opinion (Raavad Mamrim 2:9, Ritva Rosh haShanah 16a), these are laws Hashem only suggested to the court, something they may wish to enact some day, as needed. But since they are suggestions, and not actually legislated until the court decided to, the resulting laws are rabbinic.
Tangentially, there are other lists of rules. Hillel made a science of derashah, which until then were used without an overall theory and system, and noted the body of existing derashah fits 7 rules. R’ Yishma’el and R’ Aqiva each broke down those rules into subcategories. Because of differences in approach, R’ Yishma’el’s exposition yielded 13 laws, R’ Aqiva’s, 19. Rabbi Eliezer ben R Yosi haGelili later suggest 32. But these are taxonomies, not differences in actual content. You will find Rabbi Aqiva using one of Rabbi Yishmael's rules (eg kelal uperat) or Rabbi Yishmael using one of Rabbi Aqiva's (such as ribui umi'ut). The latter is far less common, but that's a topic in itself.
In any case, I mention this debate as an opportunity to point out that it is not evidence that derashos necessarily post-date Sinai, as the rules discussed describe sets of existing "data" after the fact.