Android 11 won't be available as a beta update for Pixel devices until May 2020. Until then, the only way to try the latest Android version is by manually installing it. Usually, this means carrier models are left out since their bootloaders are locked, but there's still a way to get it done.
Google just released the first version of the next major Android update. Early Android 11 builds will only available as developer previews, so you can't just sign up for the beta program and install the update as an OTA for the first couple months. That said, it's still pretty easy to get.