Let’s start with the definition of both and implement it in an example.
Processors are external parties that process personal data on behalf of the controller (in this case the website owner), pursuant to a written contract. Personal data remains your responsibility and under your supervision.
Third Parties are all other parties with whom you share personal data. The difference with Processors is that you have not signed a Processing Agreement with them, and you are no longer responsible for the personal data, or under your supervision.
Is Google Analytics a Processor or Third Party?
Google Analytics is considered a ThirdParty service because they can process the personal data they receive from your website visitors for their own purposes.
if you sign the Processing Agreement with Google, and the data is being gathered by using cookies on your own domain (first-party cookies) you can consider Google a Processor. You yourself stay responsible for what happens to the data you share with Google.
How? Read below articles for more information: