PowerShell uses .NET objects everywhere. Anything is represented as .NET object, and .NET objects come with useful built-in methods. However, for string manipulation you do not need to look for sophisticated external commands as they are built right into strings.
Here are a couple of useful examples:
"Hello".ToLower()
"Hello".ToUpper()
"Hello".EndsWith('lo')
"Hello".StartsWith('he')
"Hello".toLower().StartsWith('he')
"Hello".Contains('l')
"Hello".LastIndexOf('l')
"Hello".IndexOf('l')
"Hello".Substring(3)
"Hello".Substring(3,1)
"Hello".Insert(3, "INSERTED")
"Hello".Length
"Hello".Replace('l', 'x')
"Server1,Server2,Server3".Split(',')
" remove space at ends ".Trim()
" remove space at ends ".Trim(' rem')
"Hello".ToUpper()
"Hello".EndsWith('lo')
"Hello".StartsWith('he')
"Hello".toLower().StartsWith('he')
"Hello".Contains('l')
"Hello".LastIndexOf('l')
"Hello".IndexOf('l')
"Hello".Substring(3)
"Hello".Substring(3,1)
"Hello".Insert(3, "INSERTED")
"Hello".Length
"Hello".Replace('l', 'x')
"Server1,Server2,Server3".Split(',')
" remove space at ends ".Trim()
" remove space at ends ".Trim(' rem')