in that diagram, all four speaker wires are connected to a different [amplifier output] terminal. the voice coils are wired in
series, the speakers are not connected together. if the speakers are 4-ohms per voice coil then the
amplifier would see an impedance of 8-ohms per channel.
any true 2-channel amp CANNOT be internally bridged. in a bridgeable stereo amp, one channel is inverted. the inverting of the second channel allows both channels to be independent (stereo) or be combined (bridged) to
power a single load.
in an inverted channel amp, the signal wire on channel-2 will be the opposite of channel-1 (in other words, if channel-1's signal wire is labelled as [ + ], then channel-2's signal wire would be labelled as [ - ]). when bridged; one channel's signal wire is [via speaker(s)] connected to the other channel's [inverted] signal wire, bypassing the 'speaker ground' and allowing the bridged output to provide full voltage to a single/mono output channel (as opposed to providing 1/2 voltage to each stereo channel).
an amplifier that is internally bridged would be a mono amp. there would be no [true] stereo option at all. although most mono amps have 2 sets of speaker connections; they both output the same audio signal. the terminals are internally wired [in parallel] to a single output, which makes it a mono output regardless of the number of speaker connections.

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