The Get-Content command returns each line from a text file as a separate string, so will give you an array (so long as you don’t use the -Raw parameter; which causes all lines to be combined to a single string).
[string[]]$arrayFromFile = Get-Content -Path 'C:\USER\Documents\Collections\collection.txt'
In his excellent answer, mklement0 gives a lot more detail on what’s really going on when you call this command, as well as alternate approaches if you’re concerned about performance over convenience. Definitely worth a read if you’re interested in learning more about the language rather than just solving this one off requirement.