[Astan]

Hello everyone, I'm taking the first steps in the world of CFD and kindly I would need advice on how to generate mesh in OpenFoam with SnappyHexMesh.
I do not mean parameter settings.
I would like to understand what is the logic to be adopted to start the mesh procedure, we start with a mesh made at random and then thickens where it is needed? How do you understand where you need to thicken?
How do you understand if the mesh is of good quality?
In general, how do you proceed?
What is the criterion for determining the size of the BlockMesh in a problem of studying a vehicle's CFD?

Thanks in advance for the time spent reading the message and for any answers