About Orthomosaic (true orthophoto) generation

Through these link Photo stitching vs orthomosaic generation and Is it possible to obtain Orthomosaic without processing Step 2, I can roughly know how to produce orthomosaic. I still have some questions. Firstly, is the 2.5 model computed by 2.5D dsm? And then it projects images on the 2.5D model to generate the orthomosaic. Is this the basic process?
Look forward to your reply. Thank you.

The mesh is generated from the point cloud (as you can see mesh is created in step 2 and DSM in step 3). The orthomosaic is generated from the DSM.