Hi John-Mark, most bridges today are built from reinforced concrete and/or steel. However, some stone or brick bridges may be built or reconstructed for historical or special aesthetic purposes, and occasionally aluminium is used as it’s lighter than steel and doesn’t need painting to be protected from corrosion (rusting).
Nearly all bridges and structures use reinforced concrete for their foundations and substructure (bits of the bridge beneath the deck). Then the deck could be all steel, all reinforced concrete, or like a bridge I designed, a mixture of both – this is called a composite structure.
There have been experiments into making bridges out of plastics but these seem to be limited to footbridges as it is a much more flexible material and could be too bouncy for vehicles or trains!
You may have seen long-span bridges such as the Golden Gate Bridge in the USA, or the new Queensferry Crossing in Scotland. The cables on these are made of steel too. Ponte Morandi was a bridge in Italy that collapsed in 2019 and it had an unusual design where the ‘cable stays’ were encased in concrete which would have made maintenance and inspection more difficult, and could contributed to it’s collapse.