How can I select one half of a mesh then mirror it ?
Inside the Symmetry menu there is the mirror option. Note that the Symmetry Line can be based off the selected model OR World. You can adjust the pivot point on the model to change the mirror point.
How can I select by mask half the mesh then delete so I can mirror ?
Why do you actually want to mask and trim half the mesh? I hear the question for the first time. You don’t need to mask anything to mirror. Do you have an example picture?