So decals in this game are sort of like potions, collectables or mission rewards in other games?
Anyway I normally see two approaches used for this sort of thing
1) It is a simple counter like any other thing, just incremented presumably after some extended play sessions.
2) Each decal is its own thing and unlocked like any unique item (presumably a table or something with all the options there). This I would expect more if you can unlock individual items and still have earlier ones to grab by completing missions or whatever but could be what appears as simple unlocks too.
The basic premise of cheat searching is you find some variable that changes in memory when something happens in a game and change it such that you have infinite of them, or a set amount, or indeed possibly zero if you want to disable it.
It is far easier when you have a simple number to search for and/or can control precisely how many things you have without changing anything else in a game.
For something that unlocks after a lot of play (which presumably means a lot of changes in memory) that is harder but not impossible. If you can somehow surrender or give up decals (if searching for money it is generally easier to spend it than earn some more) then that could make life easier but I doubt you can.
Your cheat search (like the first image) then gives you a bunch of options of types of searches you can do, and they will be much as they say -- assuming you are not searching directly for a number (if you have 50 potions then the hex equivalent of 50 likely is somewhere in memory in many games, harder to search for a life bar that has no numbers though so not all will be number searches) then you have things like changes, greater than, less than, ranges and possibly a few others. These options do much as they say -- a change search will compare the state of memory before to the state of memory now and note anything that changed during that time, greater than will look for numbers that increased in size (if you are using potions and the number of them is decreasing then outside of anti cheat protections the potion count tends to go down)
The size value is much as it says and it will treat the data it looks at as however many byte chunks, or maybe types in some other cases (see floating point numbers and signed numbers). This is generally a bit of an educated guess -- many new cheat makers will start searching for something like health using a just started a game character that maybe only has a small health value compared to a level 99 tank with a bunch of addons. If your health starts at say 30 and most enemies do 1 damage a pop then you might do a low value search and come unstuck as you assumed it was only an 8 bit value (aka a byte, covers values between 0 and 255) where end game might see you have thousands of health (likely 16 bit, aka 2 bytes where you go between 0 and 65535) and thus your 8 bit hold this value might be missing a very big trick. Programmers tend to shy away from using too large numbers as they take extra effort in programming to handle (when you hear a system is a 32 bit system then that generally means 32 bits is the largest value that the processor reasonably operates with, and in the case of ARM processors like the 3ds ones then 32 bits might even be higher than is ideal for some aspects), though they will also tend to want to use the same types of numbers (adding a float to a signed integer is not as easy as adding two floats or two signed integers or two unsigned values).
For something like this decal stuff if you do have to play a lot of game to unlock it then I have two approaches I like to use to make life a bit easier.
i) Use a savestate. Make a savestate just before you unlock it, and thus you can compare things more easily and do same value search (search one moment while it is not unlocked and then again while it is still unlocked -- anything like time decreasing, animations on screen, background sounds... that caused a change and a false positive in a normal search should now be eliminated).
ii) Ignore the decals or hard thing and go for something you can control. Usually close nearby to whatever is easy to find is whatever is hard to find -- say your luck value in a RPG raises once every 10 levels or something but attack can be changed happily all day long then find attack and likely nearby (or some fixed distance away) is the luck value for that character. If in another RPG I want the end game sword that needs me to get 90% of the way through a game and 20 hours of sidequest to get I could do that (maybe with turbo mode, infinite health and whatnot to speed things up) but I could also figure out what infinite first starting town daggers are and chances are then it is either a item identifier number (item 1, count 10, item 2, count 3, item 45, count 6...) and you can play around until you find the thing you want (and maybe also some stuff the devs cut or left as bonus hidden secrets) or alternatively it is all a big table (this byte in the table represents item 1, this item 2, whatever the value is in that location then happens to be how many the party has in inventory). I don't know what the equivalent of this will be for your game here but something to contemplate.
