In simple terms it's an "expansion joint/strip".
Just like the ones you see/drive over on highway bridge overpasses.
The fuselage is made of aluminium.
Bolted to and below the fuse, is the structure/skeleton that exists to support all the lower belly fairing area & the fairings themselves and this is made from titanium(weight saving).
They have different expansion rates and due to the sheer size an expansion joint is required to prevent buckling.
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