What you are looking for is called RHI, RenderHardwareInterface.
Each graphics api that UE support has implements a RHI with api specific calls.
D3D11 is located in Source/Runtime/Windows/D3D11RHI
D3D12 is located in Source/Runtime/D3D12RHI
OGL is located in Source/Runtime/OGLDrv
Vulken is located in Source/Runtime/VulkenRHI
Also look at FDynamicRHI, this is the base interface that is implemented by the RHIs above, each function of this interface is implemented with api specific calls.
Beyond that you have the resources (Textures, Buffers ect.) they are also api specific and their base classes are located in Source/Runtime/RHI/Public/RHIResources.h
The api specific resources are derived from FRHIBuffer,FRHITexture… and the contain the wrapper of the api specific resource that in it’s turn holds the native api resource.
When unreal starts up it creates the requested DynamicRHI and stores it as a pointer to the base interface and from this point you call the functions of that.
There are some rare cases when the DynamicRHI dose not implement some feature of the api that you need or you work with middleware that needs access to the native api you can call the api directly, but those cases are rare (one of them can be found in the oculuse plugin: Plugins\Runtime\OculusRift\Source\OculusRift\Private\OculusRiftRenderD3D12.cpp, Plugins\Runtime\OculusRift\Source\OculusRift\Private\OculusRiftRenderD3D11.cpp, Plugins\Runtime\OculusRift\Source\OculusRift\Private\OculusRiftRenderGLcpp).