Copy full system and data partitions of Oneplus 3T to computer
Devices of system and data partitions on Oneplus 3T are the following:

/dev/block/dm-0
/dev/block/dm-1


To copy them to computer run (smartphone must be rooted):

adb pull /dev/block/dm-0 dm-0.img
adb pull /dev/block/dm-0 dm-1.img