+1LAXOGG.
You don't need to clear customs/immigration. UA buses terminate at arrivals terminal. QR buses do the whole loop. When your UA bus drops you off inform the attendant at the sliding doors that you need to get to the transit terminal. The next loop bus will take you there.
Should't take more than 30 minutes to do offload, transfer and security (often empty, sometimes rammed). If you need to get a boarding pass there is a desk to the right after security.
Very straightforward MR 
.