Pretrained model

[Lorenzogasparini1975]

Hi, I'm new in the AI world and in every project that I had always seen that there are pretrained model, which is very strange for me. I thought that in this projects we can access modify all the deep learning network and create our own model, understand and test how It works.
I apologize for my ignorance, but I don't understand why to run this projects I have to download a model and I don't have to create it running the code

Thx