There are a lot of pictures like this one, where one line output is being split for several headphone outputs through some device. So my question is: what is this device, what is it called or what device can I use for the same purpose?
It is an amplified headphone distributor. Sweetwater.com has many. There is a cheap one for $30 but most are a $200-300 and up. If you get an inexpensive one, make sure it’s powered - otherwise it can only lower outputs to match the most efficient (loudest) headphones being used. If you are mixing on headphones, you will be at the mercy of the quality of the amplifier in the piece. That’s not as much of a concern for recording, however.