I'm going to try my best to explain it since english isn't my first language and also because I got this cheat directly from someone else. As far as I know, you will need to spend about 2$ every few weeks since the values reset and you will need enough plays to find the value and edit it. But someone who's much more experienced than I am could potentially find the value without spending anything. Regardless:
You're going to need CFW on your system (obviously) along with BOOT NTR and the CTRPluginFramework which lets you create cheats yourself (link:
https://gbatemp.net/threads/ctrpluginframework-blank-plugin-now-with-action-replay.487729/)
With BOOT NTR on, enter Nintendo Badge Arcade and get 10 plays for 2$. Open up CTRPluginFramework by pressing minus, press "Search", press "Reset" if applicable (I forget if it's also Reset the first time you open it up), choose "Unknown search" on search type and then press "Search" again. The hits that it gets is gonna take some time but wait it out each time.
Now, everytime you use up a play in the arcades (eg 10 -> 9), open up the menu again and keep the search type as Unknown search, but select "Smaller than" on scan type and press Search again. Now as far as I've seen the value for plays is "3646C64C" and on the menu it'll show as "New Value: 0000000X - Old Value: 0000000X" which X is the number. Keep doing the above until you see this value specifically since there is another value that displays the same but it has nothing to do with plays.
After you find this value, select it with the D-Pad, press X, press Jump in editor and then change the 2 numbers to your liking in hex code (I do 32 for 50 plays), then exit the plugin and you should have unlimited plays as long as you keep changing the number everytime you're about to run out. The values you've discovered save every time so thankfully you don't need to search for them every single time you enter the badge arcade, but if you open up the editor and all you see is FF FF, it's time to repeat the entire process.
I'll put a video link below sent to me by the person who taught me this in order to maybe help you better visualize it since the video is too large to upload here, but I hope this helped regardless
https://mega.nz/file/yMVxGYhQ#tZK12bzq9mYewbwdj4eKkQ82Locl8uNHOM_TDGVmcmE