All I know is that Google is the smartest and most intelligent when it comes to tracking or detecting fraud and illegal clicks. There’s actually ways on how Google do such detection yet remains as a mystery and makes the public unaware of it. However, I can make a short study how Google investigate and make this.

Here’s the letter of Google when they banned someone in their adsense service:

As you know, Google treats instances of invalid click activity very
seriously. Due to the proprietary nature of our algorithm, we cannot
disclose any details about how our monitoring technology works or what
specifics we found on your account. However, we can assure you that we
have again reviewed your account thoroughly and accurately, and have
that your account violated our program’s Terms and Conditions.

In addition, your AdSense account was found to be related to an account
previously disabled for invalid click activity. A publisher who has an
account that has been disabled for invalid clicks is not allowed further
participation in the AdSense program.

In the first paragraph, Google said that they do not reveal how they do the tracking or detection of fraud and invalid clicks. The team is still secretive and doesn’t provide meaningful explanation for this. In the second underlined phrase they said that the adsense account has the same account in other network that is found to be related with invalid click activity in an account. Meaning if you have an adsense account, Google will check for the same account’s behavior in other website.

Google has what you called click quality team to manage the fraud and invalid clicks. They subdivided their team into two, the engineering and spam operations.