I’m having a problem with my terrain, it’s not rendering from underneath, iv’e also noticed this with any other plane I add into unity or model in blender, is this normal or is there a technical fix for this?
This is due to Back-Face Culling.
You'll have to create a shader and set "Cull Off" in the SubShader section: See Unity Documentation on shaders.
Be warned that the normals will be inverted on the back side and will cause lighting to be incorrect on two-sided lit materials.
If you need your surface to be lit correctly on both sides you will need to either duplicate the geometry and flip the faces, or apply two materials to the mesh; one regular and one custom shader that both inverts the culling and negate the normals to render the mesh twice (once for front faces and once for the back faces).