Understanding of the material in week 3

Guys, I’m getting a lot of frustration with the week 3 material, as it seems that there are only formulas, and some steps are missing. For eg. de the derivative for the (Y-Y hat), can’t really understand why is only y hat taken into account.
Do you experience the same? Please share how do you find it and what’s your approach.

For the derivative calculations, some intermediate steps may have been omitted, but IIRC the information needed to connect those dots was presented at some point during the course. Is there something specific regarding the y minus y-hat that you have questions about? I’m not sure what you meant by only y-hat being taken into account.