Programming Minecraft Villagers?

Can I somehow adjust that they only sell certain things or do completely different things, and how can I change the background music so that I don't hear the standard but e.g. Japan moderate or something?

So first of all: you can do that without further mods or plugins, no no. And to the second question: As far as I know, the background music can't be changed without further ado, otherwise just google it and you will find something 😉

Can I somehow adjust that they only sell certain things or do completely different things

For example, on McStacker you can spawn a villager who will trade what you want (there are many other websites as well)

how can I change the background music so that I don't hear this standard but e.g. Japan moderate or something?

You can do this with a resource pack

You just have to replace all the background music with the Japan music and name it as the background music files were named