To summarize: the major difference is that Arch Linux gives you the latest versions of all programs and packages. You can update anytime, and you’ll get the latest versions every time for all programs
Debian follows a stable release model. Suppose you install debian 12 (bookworm). The software versions there are locked, and they’re usually not the latest versions. For example, the Linux kernel there is version 6.1, whereas the latest is like 6,9 or something. Neovim is version 0.7, whereas the latest is 0.9. Those versions will remain this way, unless you update to, say, debian 13 whenever it comes out. But if you do your regular system updates, it will only do security updates (which do not change the behavior of a program).
You might wonder, why is the debian approach good? Stability. Software updates = changes. Changes could mean your setup that was previously working, suddenly isn’t, because now the program changed behavior. Debian tries to avoid that by locking all versions, and making sure they are fully compatible. It also ensures that by doing this, you don’t miss out on security updates.