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A lot of documentation may recommend that you start with the old APIs simply on account of them being easier to get started with. Direct3D solved this problem for beginners by offering a companion library (DirectX Tool Kit) that provides simpler drawing APIs and pre-written shaders which can be freely mixed with raw Direct3D 11 usage as your expertise grows. The wider OpenGL community has mostly stuck with the Compatibility Profile for beginners, unfortunately, which is problematic as again there are systems that do not let you mix old OpenGL APIs with the newer ones. There are unofficial libraries and tools for simpler rendering on the new OpenGL with varying levels of features and target use cases and languages (MonoGame for .NET users for instance), but nothing officially endorsed or widely agreed upon.

A lot of documentation may recommend that you start with the old APIs simply on account of them being easier to get started with. Direct3D solved this problem for beginners by offering a companion library (DirectX Tool Kit) that provides simpler drawing APIs and pre-written shaders which can be freely mixed with raw Direct3D 11 usage as your expertise grows. The wider OpenGL community has mostly stuck with the Compatibility Profile for beginners, unfortunately, which is problematic as again there are systems that do not let you mix old OpenGL APIs with the newer ones. There are unofficial libraries and tools for simpler rendering on the new OpenGL with varying levels of features and target use cases and languages (MonoGame for .NET users for instance), but nothing officially endorsed or widely agreed upon.

OpenGL's immediate mode would immediately submit vertices to the graphics pipeline back in the old days. This worked well when the speed of the hardware that rendered vertices was roughly on par with the speed of the CPU generating the vertex data. OpenGL back then just offloaded the triangle rasterization and not much else.

There are various other APIs that have been deprecated or obsoleted in OpenGL over the years. The so-called fixed-function pipeline is another such piece. Some documentation might still use this pipeline or mix with with the programmable pipeline. The fixed-function pipeline comes from the olden days when graphics card hard-coded all of the math used to render 3D scenes and the OpenGL API was limited to setting some configuration values for that math. These days the hardware has very little hard-coded math and (just like your CPU) runs user-supplied programs (often called shaders) instead.

Documentation you're finding may not even be for OpenGL but may be for one of the other similar APIs. OpenGL|ES 1.x had fixed-function rendering but did not have the OpenGL 1.x APIs for vertex submission. OpenGL|ES 2.x+ and WebGL 1+ do not have any fixed-function features at all and there is no backwards compatibility modes those APIs.

OpenGL's 1.x API would immediately submit vertices to the graphics pipeline back in the old days. This worked well when the speed of the hardware that rendered vertices was roughly on par with the speed of the CPU generating the vertex data. OpenGL back then just offloaded the triangle rasterization and not much else.

There are various other APIs that have been deprecated or obsoleted in OpenGL over the years. The so-called fixed-function pipeline is another such piece. Some documentation might still use this pipeline or mix with the programmable pipeline. The fixed-function pipeline comes from the olden days when graphics cards hard-coded all of the math used to render 3D scenes and the OpenGL API was limited to setting some configuration values for that math. These days the hardware has very little hard-coded math and (just like your CPU) runs user-supplied programs (often called shaders) instead.

Documentation you're finding may not even be for OpenGL but may be for one of the other similar APIs. OpenGL|ES 1.x had fixed-function rendering but did not have the OpenGL 1.x APIs for vertex submission. OpenGL|ES 2.x+ and WebGL 1+ do not have any fixed-function features at all and there is no backwards compatibility modes for those APIs.

A lot of documentation may recommend that you start with the old APIs simply on account of them being easier to get started with. Direct3D solved this problem for beginners by offering a companion library (DirectX Tool Kit) that provides simpler drawing APIs and pre-written shaders which can be freely mixed with raw Direct3D 11 usage as your expertise grows. The wider OpenGL community has mostly stuck with the Compatibility Profile for beginners, unfortunately, which is problematic as again there are systems that do not let you mix old OpenGL APIs with the newer ones. There are many unofficial libraries and tools for simpler rendering on the new OpenGL, but nothing officially endorsed or widely agreed upon.

A lot of documentation may recommend that you start with the old APIs simply on account of them being easier to get started with. Direct3D solved this problem for beginners by offering a companion library (DirectX Tool Kit) that provides simpler drawing APIs and pre-written shaders which can be freely mixed with raw Direct3D 11 usage as your expertise grows. The wider OpenGL community has mostly stuck with the Compatibility Profile for beginners, unfortunately, which is problematic as again there are systems that do not let you mix old OpenGL APIs with the newer ones. There are unofficial libraries and tools for simpler rendering on the new OpenGL with varying levels of features and target use cases and languages (MonoGame for .NET users for instance), but nothing officially endorsed or widely agreed upon.