1) OpenGL is a (now legacy spec) and DirectX is an API on Windows. OpenGL spec is implemented by your GPU driver.
2) DirectX is a collection of multimedia APIs on Windows. OpenGL is just graphics. Direct3D is one of those APIs.
3) Thirdly OpenGL and Direct3D (before version 12) are pretty much the same in they can do. The code is pretty similar IIRC (though it been some time since I've done Direct3D programming).
4) Devs use DirectX because there is a full set of APIs for the target platform. Linux and Mac aren't typically targeted when making a game. Mac and Linux sales have been a very small percentage historically.
1) OpenGL is a (now legacy spec) and DirectX is an API on Windows. OpenGL spec is implemented by your GPU driver.
2) DirectX is a collection of multimedia APIs on Windows. OpenGL is just graphics. Direct3D is one of those APIs.
3) Thirdly OpenGL and Direct3D (before version 12) are pretty much the same in they can do. The code is pretty similar IIRC (though it been some time since I've done Direct3D programming).
4) Devs use DirectX because there is a full set of APIs for the target platform. Linux and Mac aren't typically targeted when making a game. Mac and Linux sales have been a very small percentage historically.