So this might not be the right thread, and I'm not looking for code or anything, but I'm curious, how does one get player heads in a GUI menu, like when you open a GUI and there's a player head in every GUI slot for each player currently on the server or something? This is just a curiosity to me, I'm not doing any development with it.
Player skulls (in 1.12 and below) are of type Material#SKULL_ITEM with a byte data value of 3. From there, the SkullMeta may be used to set the owner based on its OfflinePlayer instance (or by name). SkullMeta#setOwningPlayer(OfflinePlayer). In 1.13+, similar process, but rather than SKULL_ITEM with byte data, it's just Material#PLAYER_HEAD with no data value at all. SkullMeta stays the same.