Hmm... self-SOLVED pseudo-questions and praise from newbies? Be warned. Don't use String™. Using the String class will cause you trouble forever.
Maybe the OP should read this recent discussion, this summary of the issues from all over the innertubes, and this cautionary tale. If you still wonder if String is useful, read this.
OP, if you want "to really help others", ask a useful question or provide a useful answer. Don't claim to solve the wrong problem for your 3rd post. If you have a 700-byte array and you convert it to a String, that allocates another 700 bytes. Using 1400 bytes for an HTTP response to extract one uint8_t is no solution -- it's unbelievably wasteful. The HTTP response should have been parsed as it was received.
And if you don't understand how to do that, ASK a question that may help others.