What does the r#""# operator mean for string literals in Rust?

Hello, colleagues! I recently started learning Rust and came across syntax that I completely don't understand. In one project I saw this code:\n\n```rust\nlet json = r#"{"name": "John", "age": 30}"#;\nlet path = r"C:\\Program Files\\MyApp";\nlet regex = r"\\d{3}-\\d{2}-\\d{4}";\n```\n\nI understand that `r` before a string is a raw string literal, which doesn't process escape sequences. But what does `r#""#` mean? Why are the hash symbols `#` needed around the quotes? And why can there be multiple of them, for example `r##"..."##`?\n\nI tried replacing `r#"..."#` with a regular `r"..."`, but the compiler gave an error. In the official documentation I found a mention that this is related to escaping quotes inside the string, but there were few examples.\n\nSpecifically, I'm interested in:\n1. How does `r#""#` work and