I noticed my house has a hole in this corner siding where wood meets the brick. Any ideas how to repair it (just some basic guidance)?
Nail or screw it back together where the two boards meet. Splitless siding nails would be ok, but pre-drill for the ones near the ends of the boards. Trim-head screws are also good. Some are self-drilling, which prevents splits.
Nail it to the wall. I can't tell how it was originally fastened, but those same splitless siding nails should do fine, or more trim-head screws.
Fill the hole at the bottom to prevent pest intrusion. Spray foam could work but is messy. I'd probably stuff it with steel wool, as I assume it doesn't need additional insulation.