There are a few hobbiests who've been working on smartphones powered by small chipsets like the Raspberry Pi Zero using off the shelf parts. There are also rat

There are a few hobbiests who've been working on smartphones powered by small chipsets like the Raspberry Pi Zero using off the shelf parts. There are also rather a lot of KaiOS smart phones with the form factor of a flip phone, although I don't know how easy to repair they are or how many of them work on US networks.