What does \_. means in Vim?

Question

what does \_. means in vim ?  explain to me character by character

Response from chatgpt:

In Vim regular expressions, \_. is a sequence that represents any character including a newline. 
Here is an explanation of each character:

The backslash \ is an escape character that is used to indicate that the following character has a special meaning in Vim regular expressions. In this case, it is used to escape the underscore character _.
The underscore _ matches any single character, but it does not match newline characters by default.
The dot . matches any character except newline characters.

Therefore, when you combine the \ and _ characters with the dot . character, you get \_., which matches any character, including newline characters

Answer 1

I prefer the explanation of the Vim doc:

\_. Matches any single character or end-of-line.
    Careful: "\_.*" matches all text to the end of the buffer!

\_x Where "x" is any of the characters above: The character class with
    end-of-line added

\_ is an atom modifier that add the new line to some atom matching like . or \s, or\d where:

• . match any character but new line \_. match any character including new line.
• \s match any space character but new line \_s match any space including new line.
• \d match any digit character and not new line \_d match any digit or new line.