how to fully erase internal ssd in mabookpro 2015

Creating a new file system will create some folders and metadata to track where storage is being used and what is free.

The Spotlight utility will automatically crate the *-V100 files/folders.

The Finder will create the .Trashes folders

TemporaryItems will get created by someone, most likely Finder

fseventsd is part of the file system event notification, which utilities such as Spotlight use to keep the Spotlight database updated as you add/change/delete files.

Also a file with zero bytes, may have a resource fork that has non-zero bytes. You can often times see the resource fork using

/bin/ls -aleO@ file.name/rsrc

where you append /rsrc to the file name, if you get not a directory, then the file does not have a resource fork.

Do realized the topic is what is left after formatting the drive.

Below will Wipe everything including the Recovery Volume

To perform this action will require booting from a Bootable Installer. and in you case choose Mojave This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Special Notation - works on Intel Based Apple Computers & May on Apple Silicon M1 CPU

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Those will be normal on a newly formatted drive.

Is this the boot SSD, or an external SSD?

If it is your boot drive, then it is going to have things like the Recovery partition, partition maps, etc... _AND_ the macOS operating system.

If this is an extenral SSD, then it will have file system metadata to track files and storage usage. This can be a small as a few megabytes, to a couple of hundred megabytes.

Also did you reformat the drive or the volume. There is a difference, as most drives have multiple partitions. Reformatting the driver takes it to bare metal, except you will almost always have a partition map at the beginning of the disk which takes up a few kilobytes.

But it is highly unlikely you have malware on the disk. If you are loosing files, and they are specific files, such as mail, then I would suspect something more mail specific, such as your email provider, especially if you use IMAP mail protocols, where all the mail is kept on the mail server.

What exactly left on the drive?

It will use some space for Disk Layout of Sectors & such.

Hmmm, something else id wrong I think, how short of 512 GB is it?

Did you try formatting it exFat once?