United Express carriers are regional airlines, which are operated based on a marketing code-share agreement with United Airlines. Each United Express carrier is an independet airline, responsible for their own operations and personnel.
I don't fly BUF-YYZ. I am not aware of any airline that operates that route. I usually drive up to YYZ the day of the flight or a day(sometimes days) prior to departure and spend time in the Mississauga area before flying out.