With the release of Minecraft v1.17.2 a number of new blocks have been introduced, copper being one of them. These blocks will spawn in old worlds but they will not spawn in areas that have already been loaded.

In minecraft, blocks are organized into chunks and only has a number loaded at a time. When you enter a coordinate at a previously unexplored chunk all of the blocks are created for the first time using the algorithm for your seed. Once these blocks have been rendered the chunk will always be just that minus any changes YOU make to it.

In order to find new blocks, including copper you must adventure to a biome that has them that is in a previously unexplored chunk which might require you to venture far. Copper is most often found in the Boreal Forest biome, the Taiga biome or the snoy taiga. Previously, drowneds dropped mainly gold as an ore but now also drop copper.