This is mostly hidden from the user normally, but recent versions of MacOS have a "signed system volume" that you cannot alter (of if you do manage to alter, the original will get restored somehow). If you open the startup disk in the Finder you should see these four folders.
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Open the System folder: everything inside there is protected. You'll see another Applications folder inside the System folder. These are the pre-installed apps that you cannot delete (AFAIK). It's part of the enhanced security, to prevent the core apps from being compromised.
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More info here.
SSV not only helps prevent tampering with any Apple software that’s part of the operating system, it also makes macOS software update more reliable and much safer.
support.apple.com