Custom dimension with pre made map in a given places

Hey!

I started a mod and I had many great ideas and plans for it, but unfortunately I discovered that I can not create a custom dimension as I wanted, so now I started to look after different solutions to fix it, but it did not help much. So my idea was that when I enter my custom dimension, the dimension will have pre made places that has different contents, bosses etc. but I do not know how to make the places (structures) load where I want or even load at all in that dimension. I tried different solutions with almost this same topic, but I did not understand anything. I am bad at coding, so if there is someone who is willing to explain in details how could I do that my structure/structures spawn in that dimension when I enter it, I would be very thankful and happy.

Thnx