Why does my computer say I have over 300gb used in apps?

That size is not just the sum of all of the applications. It is also the sum of all data created with the app.

You have several apps that create large data files.

Would start by purging the application CleanMyMac which may part of the overall issue

Any Third Party Applications that will interfere with the normal operation of the OS,  is an invitation for disaster and comprising the Operating System

Certain Applications maybe available on the Apple Apps Store - this only means the Developer is prepared to pay Apple a portion on each sale. What the Application may do to the computer is up to the User to check this out before purchase

Any of the below should be removed as per Developers Instructions 

This will include CleanMyMac

Read some of the posting and arrive at your own conclusions.

Even after removing the offending application as per Developers Specific Instruction, a re-installation of macOS via the Recovery Mode may or may not be required.

This would effectively replace any removed or corrupted elements of the User Account ( Home Folder ).

CMM is considered by most of the experienced users in these forums to be akin to malware. It should be uninstalled according to the developer's instructions.

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Take a closer look at the list of apps that are installed on your Mac. Do you recognize all of them? Do you use all of them? If no to either of these, you may want to consider deleting ones that you don't need. I would start by removing any "cleaner" or anti-virus apps as neither of these are required on Macs.

In the image you provided, you can see that Adobe apps take a considerable amount of storage space.

BrickMyMac is destructive. Likely the cause of every problem you have.

I downloaded iMazing and found it was storing massive backups of my iPhone.

When you told it to back up your iPhone onto your Mac, what did you expect it to store? I think my iPhone is bigger than the SSD on my MacBook.

This will include CleanMyMac

Read some of the posting and arrive at your own conclusions.