The Levites are set apart and belong to YHWH. The number of the firstborn of Israel are redeemed by the number of Levites. If the firstborn of Israel exceed the number of Levites, they are redeemed with money and the redemption money given to Aaron and/or his descendants.
Num 3:45 'Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be Mine, even the L-RD'S.
47 thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them - the shekel is twenty gerahs.
51 And Moses gave the redemption-money unto Aaron and to his sons, according to the word of the L-RD, as the L-RD commanded Moses.
Levites = Gershonites, where they camp, what they were in charge of and what they carried.
Num 3:23 The families of the Gershonites were to pitch behind the tabernacle westward;
25 and the charge of the sons of Gershon in the tent of meeting the tabernacle, and the Tent, the covering thereof, and the screen for the door of the tent of meeting,
26 and the hangings of the court, and the screen for the door of the court - which is by the tabernacle, and by the altar, round about- -and the cords of it, even whatsoever pertaineth to the service thereof.
Num 4:24 This is the service of the families of the Gershonites, in serving and in bearing burdens:
Read Numbers 4:24-28. Ithamar, son of Aaron, was over the charge of the Gershonites.
Levites = Kohathites, where they camp, what they were in charge of and what they carried.
Num 3:29 The families of the sons of Kohath were to pitch on the side of the tabernacle southward;
31 and their charge the ark, and the table, and the candlestick, and the altars, and the vessels of the sanctuary wherewith the priests minister, and the screen, and all that pertaineth to the service thereof;
32 Eleazar the son of Aaron the priest being prince of the princes of the Levites, and having the oversight of them that keep the charge of the sanctuary.
Eleazar is mentioned here with the Kohathites, because he is a Kohathite. However, he will camp to the east of the Tabernacle with Aaron's lineage, while all other Kohathites camp to the south of the Tabernacle.
Num 4:4 This is the service of the sons of Kohath in the tent of meeting, about the most holy things:
Read Numbers 4:4-15 and Numbers 4:18-20. The sons of Aaron, were to cover all the most holy things before the Kohathites, "lest they die", could come in to carry.
Levites = Merarites, where they camp, what they were in charge of and what they carried.
Num 3:33 Of Merari was the family of the Mahlites, and the family of the Mushites; these are the families of Merari.
35 the prince of the fathers' house of the families of Merari being Zuriel the son of Abihail; they were to pitch on the side of the tabernacle northward;
36 the appointed charge of the sons of Merari being the boards of the tabernacle, and the bars thereof, and the pillars thereof, and the sockets thereof, and all the instruments thereof, and all that pertaineth to the service thereof;
37 and the pillars of the court round about, and their sockets, and their pins, and their cords.
Num 4:29 As for the sons of Merari, thou shalt number them by their families, by their fathers' houses;
Read Numbers 4:29-33. Ithamar, son of Aaron, was over the charge of the Merarites.
High Priest, where they camp, what they were in charge of and what they carried.
Num 3:38 And those that were to pitch before the tabernacle eastward, before the tent of meeting toward the sunrising, were Moses, and Aaron and his sons, keeping the charge of the sanctuary, even the charge for the children of Israel; and the common man that drew nigh was to be put to death.
Anyone besides a descendant of Aaron to enter the holy of holies was to be put to death.
Num 4:16 And the charge of Eleazar the son of Aaron the priest shall be the oil for the light, and the sweet incense, and the continual meal-offering, and the anointing oil: he shall have the charge of all the tabernacle, and of all that therein is, whether it be the sanctuary, or the furniture thereof.'
Eleazar carried the oil for the menorah, the incense, the meal-offering, and the anointing oil.
He was in charge of the entire Tabernacle, but these are the specific things which he, himself, carried.