Since I installed macOS Big Sur the Safari app cannot be opened, and I can’t get current email

I understand the Safari app cannot be opened. What happens when you try? Does the Safari icon just "bounce" in the Dock, or...?

Try restarting your Mac in the usual manner. If the problem persists after restarting normally, please determine if the same problems occur in "Safe Mode": How to use safe mode on your Mac - Apple Support.

I understand Safari is crashing.

Reinstalling macOS is justified: How to reinstall macOS - Apple Support.

Back up your Mac as a matter of course. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.

An inability to receive email messages may or may not be related. For that problem, read If you can't send or receive email on your Mac - Apple Support. You probably need to reinstall macOS anyway though, so do that first.