The Rambam, before starting Hilchos Ishus writes
הלכות אישות. יש בכללן ארבע מצות, שתי מצות עשה, ושתי מצות לא תעשה. וזה הוא פרטן:
.א: לישא אשה בכתובה וקידושין
Does the Rambam hold that Ketubah is דאורייתא?
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The Rambam writes (Ishus 10:6):
וחכמים הם שתיקנו כתובה לאישה, כדי שלא תהיה קלה בעיניו להוציאה
The sages are the ones who established a Kesuba for a wife, in order that he should not regard it as easy to divorce her.
He also doesn't list the requirement of Kesuba in Sefer HaMitzvos as the Mitzvah of Kiddushin.
As to why he terms it that way, Rabbi Moshe Shmuel Shapiro suggests that Kiddushin alone, without the Kesuba, is incomplete as it doesn't permit the husband to the wife. Even though the issue is Rabbinic, the Mitzvah is to make marriage possible, not to just do the Kiddushin.
The idea of a kesubah is what you are obligated to each other this If you look in the rambams mishnah tirah hilchos ishus perek yud beis halachos aleph and beis includes 10 things 3 of which are midioraisah including cloth food and relations mentioned in shemos chof laeph posuk tes and yud 7 are midirabanan