You sir are a hero

You'll have to use the ingame LUA API.

Everything you need to know is documented here:
https://gtm.steamproxy.vip/workshop/discussions/18446744073709551615/?appid=246920

The bombenauts website also contained the API information. im not sure if the website is still up tho...

In theory you should be able to look at the source code of the custom gamemodes they are plain lua text files afaik. Im just not sure where they are saved... Especially on Windows..

I've gone ahead and just uploaded the source of angry pinata to pastebin real quick so yea...

https://pastebin.com/LgHLL9bn

yeah how was this created?

Can you possibly make a one player gamemode?

How did you even create this?!