You can use "+" with ">" to open a file for reading and writing, and the initial status is that the "file pointer" (the internal element of the file handle structure that says where the next read or write will occur) is set at the start of the file. (This means that if you write before reading, you'll replace existing content, if any.)

You can use ">>" to write to a file in append mode, and the initial status is that the file pointer is set at the end of the file, so if there is any existing content, your write operations will show up after that.

Using "+>>" is nonsensical because file access begins with the file pointer at EOF, where there's nothing to read. You can get acquainted with the "tell" and "seek" functions, to query and control the file pointer, but that gets complicated, and I don't recommend it.