Magento2 authentication

Hi everyone, I have started building an online store in magento2 and i have gone through modules and folder structure and APIs. One thing i am not able to understand is how magento2 is authenticating every request from web frontend. Is there any middleware which is intercepting every request and authenticating or any other way ? I have to add some functionality while authenticating. My front end is mobile application and i will get JWT and need to decode and verify that.

Any help will be appreciated.