MacOS 10.15.4 breaks everything including finder (iMac 15,1)

After logging in, I can see my desktop background and dock. An error message comes up saying Finder quit unexpectedly. Trying to open Finder results in a pop-up saying 'The application "Finder" Can't be opened.'

Problems:

• Most Apple apps don't seem to open (Finder, System Preferences, Safari, Photos, Notes, Music, App Store).
• No WIFI networks are visible or connected.
• Taking a screenshot with command+shift+3 shows the preview in the bottom right corner but results in the spinning colour wheel if the preview is clicked.
• Spotlight doesn't open.
• The top menu bar is only present when a third-party app is open. Clicking on the Apple logo presents the usual options. 'About this Mac' does nothing. This makes it impossible to tell if I have actually updated to 10.15.4. I just think I have because my iMac updated overnight and my MacBook Pro did not and my MacBook is running 10.15.3.
• Pressing 'option' and clicking the Apple logo shows 'System Information' as an option but it does nothing.
• Restarting normally or in Safe Mode does not fix any of these issues.

Things that do work:

• Terminal opens
• Some third-party apps open and work as long as they don't need WIFI.
• MATLAB (coding app) works. I can search the file system to open MATLAB files. I can run a MATLAB program that requires the reading of .csv files.
• Bluetooth keyboard and graphics tablet connect and work as normal.

Please help. My iMac has suddenly become unusable with this Catalina update.