Default (and possibly architecture dependents) HAL modules go here.

libhardware.so eventually should contain just the HAL hub (hardware.c), everything in it should be rewritten as modules.

Modules are .so in /vendor/lib/hw/ and have a well defined naming convention:

/vendor/lib/hw/<*_HARDWARE_MODULE_ID>.<ro.product.board>.so
/vendor/lib/hw/<*_HARDWARE_MODULE_ID>.<ro.board.platform>.so
/vendor/lib/hw/<*_HARDWARE_MODULE_ID>.<ro.arch>.so
/vendor/lib/hw/<*_HARDWARE_MODULE_ID>.default.so

They also have a well defined interface which lives in include/hardware/.

A module can have several variants: "default", "arch" and "board", and they're loaded in the "board", "arch" and "default" order. The source code for the "board" variant, usually lives under partners/...

The source code for "default" and "arch" would usually live under hardware/modules/.