BYU (24-4) will meet Long Beach State (23-8) on Saturday night (7 p.m., BYUtv) in the championship match.
"I mean, exhilarating," Patch said in describing the comeback. "A lot of it is the environment we play in and how our crowd supports us and gives us that extra lift."
UCLA (21-11) took the first two sets, and appeared to be in total control before the Cougars found a spark and rallied.
"I can't be more proud of how the boys stayed in the match and stayed in the moment," McGown said.
In the decisive fifth set, BYU broke from a 9-9 tie and scored the set's final four points to get the win, after having been swept by UCLA in Los Angeles earlier this month. Patch and Young combined for the match-winning block as the Cougars took the final set 15-10.
"We kind of put a smile on our faces again and said, 'Hey, we kinda got some work to do again,' " McGown said, when asked what he told his team at the break when it trailed 2-0.
With Sander struggling, Rivera gave the Cougars a much-needed lift, and finished with a team-high 19 kills. Gonzalo Quiroga had 22 kills for UCLA.
"I was just fired up to play UCLA," Rivera said.
The Bruins almost closed it out in the fourth set, jumping to leads of 3-0 and 7-1. But BYU came alive behind the play of Andrew Heap, who came in off the bench for starting setter Ryan Boyce, and Rivera. The Cougars won the set 26-24 on a kill by Patch.
The Cougars took the third set, 25-18, but only after leading 24-13 and watching the Bruins come roaring back.
Long Beach downed UC Irvine 3-2 in the first semifinal and will be another tough test, McGown said.
"The fact we had to play five against UCLA didn't surprise me," McGown said. "Every match in this league can be like that."
No. 1 BYU 3, No. 4 UCLA 2
O Trailing 2-0, BYU stages an improbable rally to beat UCLA 3-2 in the MPSF semifinals.
• BYU and Long Beach State which upset UC Irvine 3-2 in the first semifinal will play Saturday at 7 p.m. in the championship match at the Smith Fieldhouse.