There are a few different ways that Unity can combine meshes together, for optimization:

• “Static Batching” - The checkbox in the Unity Player Settings. Can be done on build or at runtime ¹⁾.
• “Dynamic Batching” - A now-outdated mode of runtime batching. Not recommended. ²⁾
• “SRP Batching” - A new runtime batcher for the SRP.

Note that none of these options actually combine meshes together. They will all result in the same number of graphics API drawcalls. Instead, the purpose of batching is to optimize the order of drawcalls to avoid changing state between calls.

Static Batching in Unity creates a Read-Only mesh named “Combined Mesh (root: <root-game-object>)”, with a Submesh for each batched renderer. Unity invokes a separate draw call for each range of indices within the combined mesh buffer.

Note that this means Unity does not actually render the entire batch in a single draw call. As they mention, truly merging all meshes would make culling vastly less effective.

Instead, the performance comes from issuing fewer material / mesh change state operations (in OpenGL at least). In a lot of ways, this ends up being more similar to SRP Dynamic Batching.

The lightmap coordinates from Lighting Data Asset are baked into the Combined Mesh UV coordinates when the scene loads³⁾. Afterwards, querying meshRenderer.lightmapScaleOffset will return the old scale and transform, even though it has been baked into the mesh. Technically, in the render pipeline, all of these meshes have a lightmapST (scale/transform) of '(1,1,0,0)'. ⁴⁾