I think the bridge is more like:
(Coming from the final C of the chorus before it)
C D/B
Bb D/A G
Bb A7 D
into verse changes for solo
The tune up until the bridge is solidly in C
C in the I chord
Am is the VI chord
Em is the III chord
G7 is the V chord
There are a couple F chords too, which are the IV chords.
My take:
Bb doesn't belong in the key of C. It feels like the bridge is kind of in D with the Bb still being out of key, but it is a funky half step above the V chord, A7. The bridge at least ends with a strong resolution to D from the A7. Either way the Bb sticks out and grabs you as soon it appears. Like it was said above (paraphrasing a couple people) the first chord of the bridge is often different and pulls you some where else - and in this ambiguously. I would say this a bridge that creates tension because it pulls you out of a strong key center that you are comfortable upon returning to.
I think you meant G/B coming out of the final C chord of the chorus.
I'd never heard this song before. It's an interesting choice to demonstrate a bridge because it certainly does elude a strong tonic.
To me, the Bb - D - G sounds like the key of G to me (with Bb being a bIII chord) because of the vocal melody.
The next phrase, Bb - A7 - D, is clearly in the key of D, with Bb being a bVI chord.
The key change back to C is kind of strange because it's neither abrupt or a clear change. He sings an "ohhh" melody over it (or at least he did in the live version I just heard) that sounds as though it's an extension of the bridge's last line, and not the start of the new verse. I didn't realize I was back in the verse until several chords in. This is all my first impression obviously, because it's the first time I've heard the song.
To illustrate a classic bridge, I'd use an early Beatles tune, like "From Me to You." That uses the classic move of starting on the minor v chord, which ends up being the ii chord in a ii-V of IV.
So, the song's in C, and when they get to the bridge ("I got arms that long to hold you"), it goes to Gm. That seems like a minor v chord in C, but then it continues to C7 and then to F, so we see in retrospect that the Gm was a ii in the ii-V of IV progression. The bridge then continues with D7-G7, an extended II-V (with a dominant II chord) leading back to C.
So the basic elements of a classic bridge are leading away from the home key and then leading back to it.