That is not my link. The forum auto links certain key words.
For cheat making I usually link up
https://web.archive.org/web/20080309104350/http://etk.scener.org/?op=tutorial
and
https://doc.kodewerx.org/hacking_nds.html (if that is down then future forum searchers would do well to search enhacklodedia as it is copied many places)
I did witter a bit about them
https://gbatemp.net/threads/how-to-create-a-cheat-code.635276/#post-10191991 and the hacking tutorial in my signature
https://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/ might contain something of interest here (if you wanted to do a full hack to see how it decodes new beyblades that would tell you which type you are dealing with in a hurry).
As above if it is within the game already (likely if others would have to play against it, though you could transmit the relevant details as part of a normal link play) then your approach would either be something akin to making an infinite potions cheat, or perhaps a play as boss cheat.
Infinite potions is a standard intro to cheat making. You start with a bunch of potions, or a bunch of money in a shop I guess, and use or buy them one at a time (if in a shop you might have to exit out to make it write it if you can undo your sales/have to confirm a deal) using the cheat search tool to scan for changes. Eventually other things happening in the background should filter out and you should end up with a number that corresponds to potions in your inventory.
One of three setups, though in most cases the first two, happens then
1) The memory locations themselves correspond to how many items you have. Next location over might then be the next kind of potion, next location a even better potion...
2) The memory locations contain an identifier and an amount value. You then fiddle with the identifiers (and amounts) to see what other numbers there bring. If 1 through 17 is the normal range for normal gameplay then other things (or bosses) might be 18,19 and 1A (hexadecimal in ROMs after all) but it could also be the end of the range, or random numbers within it. Next and last numbers being my first choice for a test but if you have to test them all then so it goes.
3) Full on custom details for the item. Usually seen more in big modern RPGs with lots of loot and options for it (stats, resistances/bonuses to elements, name, picture maybe, associated fluff text...) but I could see it for a beyblade in some senses as it is a group of custom things that may grow (or at least the monster within, can't remember the name gains levels). This would also be the case if it truly was downloaded custom from the servers and is not within the normal game.
There are certain things to look for to tell whether a game uses 1, 2 or 3 above before you go in for it but realistically at this point it does not matter. Sometimes 1 and 2 co exist with 1 being key items and 2 being inventory fodder you can buy-sell.
Equally do find the simple items -- no cheat maker does the epic 20 hour sidequest to get one final end game weapon (maybe only one appearing in the whole game with no option for more) when the first shop will sell you as many starting daggers or whatever as you like (you are a cheat maker so give yourself infinite gold) and then move sideways.
