כִּ֣י לִ֤י כָל־בְּכוֹר֙ בִּבְנֵ֣י יִשְׂרָאֵ֔ל בָּאָדָ֖ם וּבַבְּהֵמָ֑ה בְּי֗וֹם הַכֹּתִ֤י כָל־בְּכוֹר֙ בְּאֶ֣רֶץ מִצְרַ֔יִם הִקְדַּ֥שְׁתִּי אֹתָ֖ם לִֽי׃
因為以色列人中一切頭生的，無論是人是牲畜，都是屬我的；我在埃及地擊殺一切頭生的那日，就使他們分別為聖歸我。
Rashi
כי לי כל בכור FOR ALL THE FIRSTBORN [OF THE CHILDREN OF ISRAEL] ARE MINE — This means: they became Mine by strict right (lit., by the line of justice), because I protected them distinguishing between them and the firstborn of Egypt, and I took them as Mine — until they sinned by worshipping the Golden Calf, but now ואקח את הלוים I HAVE TAKEN THE LEVITES in their stead.
Sforno
כי לי כל בכור, I had already owned the firstborn whose duty it had been to perform the tasks now given to the Levites. The reason that the firstborn had originally been charged with the duties was that they were the most distinguished members of their respective families and one delegates those to represent one before one’s lord and master, before G’d.
Chizkuni
ביום הכותי כל בכור, “on the day when I smote every firstborn. On a different occasion, in Exodus 12,29, the smiting of the firstborn was not described as having occurred on “a day,” but as having occurred at midnight. We see from here that every time the Torah uses the term: יום, “day,” it speaks of a 24 hour period, i.e. night and day.