The ancestral tribe of the Levites, however, was not counted along with the others. The LORD had said to Moses: “You must not count the tribe of Levi or include them in the census of the other Israelites. Instead, appoint the Levites to be in charge of the tabernacle of the covenant law—over all its furnishings and everything belonging to it. They are to carry the tabernacle and all its furnishings; they are to take care of it and encamp around it. Whenever the tabernacle is to move, the Levites are to take it down, and whenever the tabernacle is to be set up, the Levites shall do it. Anyone else who approaches it is to be put to death. The Israelites are to set up their tents by divisions, each of them in their own camp under their standard. The Levites, however, are to set up their tents around the tabernacle of the covenant law so that my wrath will not fall on the Israelite community. The Levites are to be responsible for the care of the tabernacle of the covenant law.” Numbers 1:47-53
The Levites had very specific duties separate from the other tribes. Their full concentration was on the tabernacle.
Aaron and his sons were the priests. Nadah and Abihu died when they tried to do a fire sacrifice their own way. Eleazar and Ithamar were the priests. Aaron was the high priest. Everyone from that family line would serve as priests.
Levi, in Genesis, had three sons - Gershon, Kohath and Merari. Each of those three tribes had specific jobs to pick up and move the tabernacle, the altar and the holy objects. They were. . .holy roadies.
They lived right next to the tabernacle, while the tribes lived around the tabernacle.
The priests received special robes to wear and were consecrated (anointed, commissioned) to do these very specific roles.