You seem to be talking about computer games, so I will say this:

• The easiest way to do this is to forego the hardware altogether. Mouse movements and keyboard inputs are trivial to generate programmatically and grabbing the screen contents is easy too. Building a software that understands the objective of the game and can react accordingly is the hard part
• If you wanted to do this outside the computer where the game runs you could emulate a HID device on another computer and forego the keyboard and mouse, but it would be only useful to avoid some anti cheat systems
• It is certainly possible to also build a mechanical system too, but adide from being a fun challenge for an experienced mechatronics engineer, I don't see the point, especially if you are a beginner

In all of the above, start with the first one these solutions can be expanded to be more complex.

What games are you looking for that would benefit from this, that don't offer computer opponents?