Most games dont make their source open or provide a scripting language/api so for most games you have to reverse engineer the game states and manually control the memory. Minecraft has a big modding community but bots can be hard to build for it, as is the case for a lot of games. The issues come when you need to basically recreate the game environment so the bot knows what to do/where to move. If the game has basic navigational tools like auto-pathing to a destination and you want to do something like make a trading bot that uses some in-game ui to trade items then thats much easier than trying to navigate a dynamic 3d world which is a very difficult challenge even for large game companies. If you really want something to dive deep into, starcraft provides an api for making bots. It might be fun to check it out and make a bot for you to play against even if you have little experience playing the game.