The laws of arba'at haminim and of building sukkot (the structure, not the holiday's name) abound. From whence are these laws derived? Are they all considered Halakhah Le'Moshe Mi'Sinai?
With regards to the Sukkah, in the beginning of his introduction to his commentary on the mishna, the Rambam explicitly uses the Halachot of a Sukkah as Halacha L'Moshe MiSinai. He uses it as an example of a Mitzvah that was given by Moshe together with its explanation. It is obvious that explanations to "Dwell in a Sukkah for 7 days" were given, to explain what makes a kosher Sukkah.
The explanations which we have received from Moses are not contested at all. Throughout all the ages, from Moses' time until the present, we have not heard of a dispute started by a Sage whether... the expression "a fruit from a beautiful tree" refers to the etrog. Nor have we heard of a dispute that the "covered tree" refers to the myrtle...
Concerning these and the like, it has been said: "The entire Torah, [both the mitzvot] in general, and all their particulars were given to Moses on Mount Sinai." Though the tradition was received and there is no dispute about it, one can also derive these definitions through the accepted processes of exegesis.
Thus, when the Talmud debates and discusses a concept and offers a suggestion...that perhaps, "the fruit of the beautiful tree" is a pomegranate...[and does not resolve the matter] until they bring proof from the exegesis of the verse..., one should not conclude that the matter was left in doubt until this point of exegesis was discovered.
Rather, from the time of Joshua onward, we saw that the etrog was the species taken with the lulav and there is no debate about that. They were merely investigating how they could find support from the Torah for the interpretation that had been transmitted.
However with regards to the details about the various kinds, The Ba'al HaTanya, based on the Rosh and the Ran, says (Orach Chayim 749:22) (my bolding):
...לפי שכל פסולים אלו לא נתפרשו היטיב בכתוב אף על פי שכתוב בתורה הדר מכל מקום לא פירש הכתוב מהו הדר ומהו אינו הדר אלא כל פסולין אלו מסרן הכתוב לחכמים
...Because all these disqualifications are not well explained in the verses; even though the Torah writes "Hadar", it does not explain what is considered beautiful and what is not. Rather, all these disqualifications were given over to the Rabbis [to decide].