The following nodes are all dereferenced from the string node not string(constant) or string(type) just string.
Concat will concatenate two or more strings into one combined string. Change the overload to Concat(string, string) to add two strings together. It will add str1 to str0.
The Compare node will give one of three ouputs depending on the relationship between the two strings.
A string is considered less then another string if it comes first in alphabetical order.
by default the comparison will be case sensitive to ignore casing you can change the overload to Compare(string, string, bool) the extra parameter is "Ignore case" this takes a true or false value. If you don't care about matching the exact casing then input a true here.
The Equals node will output true if the two strings are equal or false if the two strings are not equal.