Hi, I am sorry to hear your S21 Ultra is freezing and crashing. When your phone freezes or crashes, it can be really nerve-racking. The good news is that this can probably be fixed without having to send your phone in for service. Performing simple actions like rebooting and updating the software are just a couple of the things you can do to prevent freezes and crashes in the future. Give these steps a try: https://www.samsung.com/us/support/troubleshooting/TSG01001416/
Additionally, you can try clearing your device's cache partition. The steps for that are:
Performing a wipe cache partition removes any temporary files that may be causing an issue with the device. All personal files and settings are not affected by this option.
- Ensure the device is powered off.
If the device is unresponsive/frozen, press and hold the Power button (located on the upper-left edge) for approximately 10 seconds or until the device power cycles.
- Press and hold the Volume Up button (located on the upper-left edge) and the Home button (located at the bottom in the middle).
- While continuing to hold the Home and Volume Up buttons, press and hold the Power button (located on the right edge) until "RECOVERY BOOTING" appears in the upper left then release all buttons.
- Select wipe cache partition.
Utilize the Volume Up/Down buttons to highlight and the Power button to select.
- Select reboot system now.
Option is highlighted, utilize the Power button to select.
Lastly, if the above two do not work, try Safe Mode and see if a third-party application might be to blame. The guide to Safe Mode is here: https://www.samsung.com/us/support/answer/ANS00062983/
Please let me know if this helps.