__load_start_overlay_text0 Load$$overlay_text0$$Base what is the usage of "$$" ?

The usage of "$$" in the context of your question is to indicate a special syntax for referencing a specific section in an object file or an executable. In this case, the syntax is used in the linker script to load the contents of a section called "overlay_text0" from the input object file to a specific memory address in the output executable. The double "$" is used to differentiate this syntax from other variables or symbols that may be used in the linker script.