Some convincing explanation

[hv0905]

File Content

PowerShell reads file line by line, which works like .NET 's File.ReadAllLine(). which means, the $myLetter is a string array.
And $myLetter.Contains("Hi"); only searches in the first child of the array.

Space after `

This is obvious. ` escape any character in pwsh. Without the space, pwsh will escape the LF, and will seize

$aAndB = $a `
    -and $b

as one expression.

But with the space, ` will escape the space instead of the LF. and will seize the codes as two expressions.
$aAndB = $a will set $aAndB to true
-and $b is invalid and will be ignored.

In powershell, theory and practice are combined: Nothing works and nobody knows why.