Create custom dimension

Hi,
I want to have a block with 3 buttons in the GUI that can be placed in the overworld. To activate a button you have to put an item in a field. Then you can press the corresponding button and a 3x3 portal appears on the right side of the block that leads to a dimension. If you press another button, you will be taken to another dimension, but the counterpart will remain in the created one. If you go into this counterpart, you will return, even if another dimension is selected. How can I do this?
Thank you very much, Laurin