each form has a construction on USCIS page , open it and follow up with each part and it will guide you, good luck.

I read the whole guide, even with my husband. it’s says nowhere who they mean

sorry , some legal assistance fill the application for you for kinda cheap price , if you are unsure you can seek their help , better than filling unsure and get delayed for too long .

@Juila H., part 4 biographic information is for the US citizen/petitioner