wiring underground.

If it's Direct Burial Wire 2 ft deep it will be fine.Pipe is good if you ever have a problem you can just pull in another wire.

I NEVER use direct burial cable but it's legal if it's 18" deep.

I've seen some in service for 50 years with no problems.

Electricians don't like it because we like access for repairs. Conduit protects the wiring and allows us to pull new wires if somthing goes wrong.

Gophers don't generally chew thru conduit.

What will the circuit be used for & where will the wire be run?
If GFCI protected before it goes into the trench then you can go to 12" for a 20a circuit
I do conduit also - just better IMO

It has to be 18" deep, even if it is in conduit? Or 12" deep? . 20 amp circuit to a garage.

Will this be the only circuit run to the garage?
If you need more then one circuit then you are better of (required?) with a small panel - or a MWBC

!2" deep if GFCI protected before it goes into the ground - without conduit
It must still be protected by conduit where it enters & leaves the trench