Is there a way to get xcode code from the App that was already in the store? have lost the original file on my MAC.

"is there a way to get the code back from the binary that's uploaded in to the store?"

No, the binary does not have the source code.

You should place your projects under version control so you don't lose your code. The following free book shows you how to use version control with Xcode:

Swift Dev Journal’s Guide to Version Control

Ah, whoops, misread that and thought it was Xcode.

No backups? No version control? That's a problem. That written, use Spotlight to search for some keywords or strings from your source code, and see if you can find it in some remaining temporary directory still located somewhere.

Xcode support tends to be fairly tied to macOS versions.

Xcode from the Mac App Store does not require membership in the developer program, but your old macOS version (10.15) means you'll need an Xcode version older than what is usually available via the app store, if your Mac can't get to macOS 13.

If the Mac App Store is where you got it, check your Previous Purchases, and see if you can download Xcode there.

You might get access to downloads of older Xcode versions with the free developer tier developer program enrollment, but I have no way to check that.

According to the XcodeReleases website, you're looking for Xcode 12.4, if you're limited to macOS 10.15 on that Mac. There are links to Apple downloads included at that XcodeReleases website, but I don't know if you'll have access to those downloads.