Usually, when we read about characters, there’s a story with an exposition, sometimes prolog and only then we can read about their actions. The story with Aaron and his sons is different. Although there’s information about them in Exodus 6:23-25, the next time that we hear about them is in our chapter- Exodus 28.
Most of the information in this chapter is about the clothes of Aaron, but also his sons are mentioned at the beginning of this chapter and in the end, as written:
“וְאַתָּ֡ה הַקְרֵ֣ב אֵלֶיךָ֩ אֶת־אַהֲרֹ֨ן אָחִ֜יךָ וְאֶת־בָּנָ֣יו אִתּ֔וֹ מִתּ֛וֹךְ בְּנֵ֥י יִשְׂרָאֵ֖ל לְכַהֲנוֹ־לִ֑י אַהֲרֹ֕ן נָדָ֧ב וַאֲבִיה֛וּא אֶלְעָזָ֥ר וְאִיתָמָ֖ר בְּנֵ֥י אַהֲרֹֽן׃וְעָשִׂ֥יתָ בִגְדֵי־קֹ֖דֶשׁ לְאַהֲרֹ֣ן אָחִ֑יךָ לְכָב֖וֹד וּלְתִפְאָֽרֶת…. וְלִבְנֵ֤י אַהֲרֹן֙ תַּעֲשֶׂ֣ה כֻתֳּנֹ֔ת וְעָשִׂ֥יתָ לָהֶ֖ם אַבְנֵטִ֑ים וּמִגְבָּעוֹת֙ תַּעֲשֶׂ֣ה לָהֶ֔ם לְכָב֖וֹד וּלְתִפְאָֽרֶת׃וְהִלְבַּשְׁתָּ֤ אֹתָם֙ אֶת־אַהֲרֹ֣ן אָחִ֔יךָ וְאֶת־בָּנָ֖יו אִתּ֑וֹ וּמָשַׁחְתָּ֨ אֹתָ֜ם וּמִלֵּאתָ֧ אֶת־יָדָ֛ם וְקִדַּשְׁתָּ֥ אֹתָ֖ם וְכִהֲנ֥וּ לִֽי”
“And take thou unto thee, Aaron, thy brother, and his sons with him, from among the children of Israel, that he may minister unto me in the priest’s office, even Aaron, Nadab and Abihu, Eleazar and Ithamar, Aaron’s sons. And thou shalt make holy garments for Aaron thy brother for glory and beauty… And for Aaron’s sons, thou shalt make coats, and thou shalt make for them girdles, and bonnets shalt thou make for them, for glory and beauty. And thou shalt put them upon Aaron thy brother, and his sons with him; and shalt anoint them, and consecrate them, and sanctify them, that they may minister unto me in the priest’s office.”(Exodus 28:1-2, 40-41)
The function of priests started 3200 years ago approximately and continues until today with the priestly blessing that will be discussed in Numbers 6:24-26.