There are two kinds of drivers: kernel drivers and libs in userspace. The second kind can be used via libhybris. Kernel drivers are supposed to be open-source due to license of Linux kernel, but Chinese manufacturers don't care about GPL.

Android kernel binary can't be used directly for SailfishOS (kernel needs recompiling with different options). So without having kernel tree source code the only option is using kernel source tree from different device and fixing the differences (different components or different pinouts, for example) by reverse-engineering stock kernel.