Variable called Output Field Separator contains string value that is used when converting an array to a string. By default $OFS = " " (a space), but it can be changed:

PS C:\\> $array = 1,2,3
PS C:\\> "$array" # default OFS will be used
1 2 3
PS C:\\> $OFS = ",." # we change OFS to comma and dot
PS C:\\> "$array"
1,.2,.3