You can try doing a factory reset and setting the phone up again. Make sure the software and all apps are up to date. Close any apps that you aren't using. Have you installed any 3rd party apps?
Mine keeps freezing the exact same way. Mostly in Chrome but also in anything you need to scroll down to read. I have tried to restart and do a factory reset. Neither have worked.
Try powering off the device and hold volume up and power it back on. Select wipe cache partition using to volume rockers and then press the power button to confirm. After that select restart device. The problem should be resolved
S20 ultra randomly freezes on me too, even in safe mode after factory reset n wipe cache partition. Waiting on stock for early life failure like tons of others.. sounds like a disaster of a phone lol