How to manufacture a phone in 7 steps: 

1- Call a Chinese phone manufacturer.
2- Give them the specs, design, quantity.
3- They give you a price. 
4- You start a kickstarter. 
5- You pay the manufacturer. 
7- They deliver the phones. 

*But someone needs to make the OS. 🤣 

 android custom rom projects work well, look at CorvusOS, amazim experience.