Phones are small, you gonna need a microscope or at least a magnifier with good lighting. BGA is not too hard but you need these laser-cut reballing stencils, i

Phones are small, you gonna need a microscope or at least a magnifier with good lighting. BGA is not too hard but you need these laser-cut reballing stencils, ideally not universal but specific to particular chips. AFAIK lead-free solder is very hard to do at home, to use leaded solder you gonna need a setup with good ventilation. If the goal is to make just a single phone, and you don’t already own all that stuff, I’m afraid the required equipment is going to be above $1000. Maybe if you live in China will be cheaper, but not for the rest of the world.