The following resources are added by the mod, but may be overridden by a resource pack. The Minecraft Wiki has a tutorial on resource pack creation, but the gist is put stuff in a similar directory as the thing you’re overriding.

owo UI

For more info about owo UI’s markup, please see their documentation on the topic.

confirm_join.xml

This is an XML markup file for the confirm join prompt that is shown to the user when a Pokémon attempts to join their party via the join on defeat mechanism. It shows a message stating that a Pokémon with the given species is trying to join the player’s team, and asks if they would like to accept or reject.