How the verification code works

I would like to ask the implementation principle of the verification code. it is necessary to verify the verification code. why do I need to obtain some spots on the verification code to interfere with visual recognition? I don't think it's okay to interfere. What is the implementation and application principle of the verification code? Can someone briefly explain this? Thank you. I feel like I have used it to verify the implementation principle of the verification code.
The verification code is necessary. why is it necessary to obtain a verification code that is similar to those posted by Baidu to interfere with visual recognition? I don't think it's okay to interfere.

What is the implementation and application principle of the verification code? Can someone briefly explain this? Thank you.

I feel like the hidden element in the html that should be used with the verification code. do you know?

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Without interfering, it is easy for others to use orc recognition technology to obtain your verification code, so that they can easily create information publishing robots.

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Enter the random number displayed on the page to allow the user to fill in the form, and the random number is written into the SESSION, the post-processing code is submitted to determine whether the user has submitted the random number and compared it with the value in the session, then ....
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But how does the server know the client browser that the SESSION variable $ _ session ['verify '] belongs?

A: Each browser client creates a unique SESSION with the server.
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