Mac suddenly inputs weird symbols (åß∂ƒ©) and when I restart it’s asking for untitled disk.

That's indicating that there's some 3rd party software that may be interfering with normal operation.

Download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Add Text button to include the report in your reply. How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

MarcS2002 wrote: suddenly those weird symbols (åß∂ƒ©) come out.

That means the machine thinks the option key is being held down. It may indicate the keyboard is damaged. Try an external keyboard.

Luis is spot on. You've installed two totally unnecessary and system interfering apps, CCleaner and Avast. There is no reason to ever run any "cleaning", "optimizing", "speed-up" or anti-virus apps on your Mac. 

This user tip describes what you need to know and do to protect your Mac: Effective defenses against malware and other threats - Apple Community   

Uninstall both according to the developer's instructions. When done you can check to see if you've removed all of the files by downloading and running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For example for Avast software you'd do the following search: 

1 - Name contains avast

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Do the same for CCleaner.

I see you installed CleanMyMac but didn't see any of it's files listed in the report. Did you uninstall it already? It is probably more dangerous to your system than the other two.

The report indicated you had a high battery count. A good app to check the status of a batter is Coconut Battery. Use it or another similar to keep tabs on the battery's capacity.

Try resetting NVRAM.

Uninstall avast.

Also you are shooting yourself in the foot by trying to fix anything by installing cleanmymac. This **** corupts everything it touches. You may end up having to do a clean install.