Like /u/someothherguyy said it depends on the cable type. There are shielded and jacketed outdoor cables that don't require PVC/EMT, but usually you'll have to terminate them yourself so there's additional tooling required.

I went with PVC because it's the easiest to work with, NEC only required it be buried 12in below the surface for residential, but I went to 24". I oversized the PVC a bit so I had room to run more cables as well. I filled the trench with gravel, ran the PVC over that, then filled over it with some more gravel. A thin layer of dirt, marking tape and then the rest of the dirt. Rent a trencher if you have hard ground.

If you're having a licensed electrician do the work of connecting house and office power then you can ask them to run the conduit for it at the same time.

it depends on the cable type, what you are willing to spend, where you live, etc

i tried to find a page that wasn't terrible for this, still kind of bad though:

https://www.gloryoptics.com/info/how-deep-does-spectrum-bury...