I am running service to detached garage apt. Here's the basics: underground service from utility's pole to meter can at rear of house. Then directly adjacent and connected by 2" pvc , second , un-used meter can. From there 2" PVC under ground to new 200AMP distribution panel in interior of garage. I anticipate using 2/0 CU to new panel. I am un-clear if the new panel needs the neutral tied to the ground or if they remain separate and the grounding to earth at the garage. The service is currently grounde w/ ground rod at meter can. I have a retired building inspector who will check out my work and I WILL bring in a licensed electrician to do the stuff that is over my head, but I would like to have all my ducks in a row to minimize cost. Thanks

You will need a ground rod (or two) at the garage/apt.

The water will also need to be bonded if the piping is metallic. If it is all or mostly pex or plastic then obviously no bond is required.

From the meter to the main panel you need three conductors. Typically the neutral can be undersized. 2/0, 2/0, 1/0 CU is typical for 200 amps.