"Aaron and his sons shall arrange it" (Exodus 27:21). What does this teaching come to say? Since it is said, "toward the face of the menorah the seven lamps shall give light" (Numbers 8:2), one might think a single priest enters to tend all seven lamps. Therefore the text says, "Aaron and his sons shall arrange." How then? Aaron and his sons arrange only one [each tends his own]. And since it is said, "the seven lamps shall give light," I might hear that they should burn forever. Therefore the text says, "from evening until morning" (Exodus 27:21). If from evening until morning, one might think he may extinguish them [by morning]. Therefore the text says, "the seven lamps shall give light." How then? "The seven lamps shall give light" from evening until morning. "Before the LORD continually" (Exodus 27:21) means that the western lamp shall be constant, for from it the menorah is kindled at twilight.
How One Priest Arranges All Seven Lamps of the Menorah
Yalkut Shimoni on Torah 378:10
(שמות כז כא) יַעֲרֹךְ אֹתוֹ אַהֲרֹן וּבָנָיו, מַה תַּלְמוּד לוֹמַר. לְפִי שֶׁנֶּאֱמַר "אֶל מוּל פְּנֵי הַמְּנוֹרָה יָאִירוּ שִׁבְעַת הַנֵּרוֹת", יָכוֹל יְהֵא כֹּהֵן אֶחָד נִכְנָס בְּשֶׁבַע נֵרוֹת, תַּלְמוּד לוֹמַר יַעֲרֹךְ אַהֲרֹן וּבָנָיו, הָא כֵּיצַד, אַהֲרֹן וּבָנָיו אֵינָן עוֹרְכִין אֶלָּא אֶחָד. לְפִי שֶׁנֶּאֱמַר (שם) "יָאִירוּ שִׁבְעַת הַנֵּרוֹת", שׁוֹמֵעַ אֲנִי שֶׁיְּהֵא דּוֹלְקִים לְעוֹלָם, תַּלְמוּד לוֹמַר מֵעֶרֶב עַד בֹּקֶר, אִי מֵעֶרֶב עַד בֹּקֶר, יָכוֹל יְכַבֵּם, תַּלְמוּד לוֹמַר (שם) "יָאִירוּ שִׁבְעַת הַנֵּרוֹת", הָא כֵּיצַד, "יָאִירוּ שִׁבְעַת הַנֵּרוֹת", מֵעֶרֶב עַד בֹּקֶר. "לִפְנֵי ה' תָּמִיד", שֶׁיְּהֵא נֵר מַעֲרָבִי תָּדִיר, שֶׁמִּמֶּנּוּ מַדְלִיק אֶת הַמְּנוֹרָה בֵּין הָעַרְבַּיִם.