Short version you don't. Anything there was largely proof of concept or requires very specific nowadays junk carts (if you are paranoid about bricking by install you definitely don't want to be updating source code to an embedded device). Similarly nitrohax
https://www.gamebrew.org/wiki/NitroHax serves much the same functionality (and 100 times better than it ever was) unless this is some kind of competition thing, in which case you have far better methods still and also not really the sort of thing we do around here. If you have a flash cart somehow without cheats (or an engine you don't care to use) there is also DSATM to hardpatch cheats into games
https://gbatemp.net/threads/ds-auto-trainer-maker-dsatm.80540/ if you can't be bothered to manually hardcode cheats in
https://gbatemp.net/threads/crackers-ds-trainer-maker-tutorial.44410/ (old and I have some better guides around but, including a small section in
https://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ ). Plus save editing, plus dumping save to use cheats on an emulator and flashing back afterwards, possibly plus save state editing if one of your carts has that, plus I don't think the DSone had a real time cheat search on it and that was the DSTwo but I think it at least had nice enable/disable if you can't be bothered to alter a more basic cheat to be button activated
https://doc.kodewerx.org/hacking_nds.html ).
As you asked though the replacement firmware with cheats baked in was fwnitro, was a wholesale replacement rather than a mod.
http://fwnitro.caitsith2.net https://www.gamebrew.org/wiki/FWNitro https://web.archive.org/web/20071226214742/http://zzo38computer.cjb.net/Nitro2/fwnitro/ for links.
Creebome
https://gbatemp.net/download/creebome.29609/ is the other main thing people consider, though the flashme site might also list Loopy's minimal firmware (which I think is original DS only anyway, and if I am not using a basic black background-white text loader for something there is a reason).
I believe it was a pokemon forum I once saw a discussion about hardcoding some cheats in (the firmware space at least on the normal models, though even the Chinese ique one might not do too much better, is too small to do an action replay a like embedded within it with a decent cheat library, more than enough to do things like put a thumb on the scales for the RNG in a single game though) but I never saw anything more. Such people would have been more than capable of doing something in that world too -- they have those quite capable of playing in assembly to do substantial changes and have frequently made tools or substantial forks to existing ones, and figured out hardware quirks for some things in the past too. Might have gone underground but eh, or at least it is the only underground hidden cheats world that never had the drama spill out or people get busted at tournaments and general pokemon hacking world is about as drama filled as it gets at the best of times.
https://cturt.github.io/ds-cfw.html https://github.com/CTurt/cturt.github.io being some links that might be interesting, sadly videos seem to have gone private at some point.
https://gbatemp.net/download/hbfirmware.29610/ for the sake of something else though I don't know much about that one.