how to create object detection model from scratch ?

[Akshay-Shingala]

hello, I am having some trouble in creating an object detection model
most tutorials and videos create models for classification to some value prediction or they retrain available pre-train models.
but how can I create an object detection model or segmentation model?
if I use a pre-train model then I can only feed them with my detection dataset how can I improve model accuracy for detecting objects and create a bounding box for it