John Gill Commentary Numbers 8:18

"And I have taken the Levites instead of all the first-born among the children of Israel." — Numbers 8:18 (ASV)

And I have taken the Levites for all the firstborn of the
children of Israel .
] Gave up his right in the firstborn, and instead of them took the Levites for his use and service: see (Numbers 3:12) .