Gfyuki

If code.txt (or whatever file) is edited and saved:

[Theory 1] The PC deletes code.txt completely and makes a new code.txt (edited version) from scratch.

[Theory 2] The PC edits part of hex of code.txt. So no delete happens.

Which theory do computers work?

Could be either – it depends on the text editor that was used.

The concept of a 'text file' isn't built into computers – each operating system may manage files differently, and each text editor may use those files differently.

In practice, you'll find text editors which have both mechanisms. Practically all operating systems allow direct overwrite of an existing file's contents, so simple editors such as Notepad usually just ask the OS to write directly into the original file, as that's easiest to implement – but risky if you lose power mid-write. So for reliability reasons, many editors deliberately save the updated data to a new file and delete the original.

(I think in-place updates are more common among hex editors, where most edits don't insert/delete bytes but only change existing locations, so a full rewrite file is not needed.)

There's even a third mode of operation – the editor might first make a backup copy of the old file, then directly write new data into the file.

It also depends on the filesystem which keeps the file. With most traditional filesystems, if a program asks to write to an existing file, the filesystem will just overwrite old data in-place.

However, some filesystems do work in "copy-on-write" mode, where any new data is always written to a different location, whether the program wants it or not. Again, this has the possible advantage of increased reliability because an interrupted change can be fully reverted.

In some filesystems (such as Btrfs or ext4) this is an optional feature; in others (e.g. log-structured filesystems) it is part of the core design.

Generally speaking, A computer will allocate the memory where the original file resides as 'deleted', but all this really means is that it won't show up in your file browser anymore, and the cells in the memory where it was written are allowed to be overwritten in future.

As to whether the new file is written into the same place is down to a number of factors, primarily the software you are using and how it is designed to make use of the memory.