Both brass and bronze are mostly copper, with small amounts of other elements added.
In the case of brass, the alloying element is zinc. There may be other elements, but the zinc is the main constituent behind the copper itself.
In the case of bronze, the alloying element is often tin, but there are lots of other alloying elements that also lead to the alloy being called bronze, like phosphorus, silicon, and aluminum.
So,
brass is copper and zinc. Bronze is copper and things that aren’t zinc.
There are a few other complications: some alloys have their own names.
For example, you don’t call an alloy of copper with five percent nickel
‘nickel bronze’, because it’s already called ‘nickel silver’. A bronze
made with copper and beryllium is called ‘beryllium copper’ rather than
beryllium bronze, for some reason.