To append all strings in a list, use a StringBuilder:

StringBuilder builder = new StringBuilder();
foreach (string s in list) {
   builder.Append(s);
}
string result = builder.ToString();

Above you also see the use of the foreach loop. For comparison, here's
how it's done with a regular for loop:

StringBuilder builder = new StringBuilder();
for (int i = 0; i < list.Count; i++) {
   builder.Append(list[i]);
}
string result = builder.ToString();

Which loop is better depends on what you want to do. One additional
advantage of the foreach loop is that it keeps track of the state of the
list you are iterating, so that if the list changes while you are
looping it, you get an exception.