When creating an mbed application, is it possible to see the code that is used for the specific device?
E.g., for the NUCLEO_WB55RG I would like to see the ST drivers that are used.
I took a look in the mbed docs but did not see anything regarding that.
If you have the source, of course you can "see" it. Check your link map to see what is included in the build or use a debugger to step "in-to" the third-party code - if the source is available it should be shown as you enter it.
Moreover any of the code included that is open-source, you will be able to access the source, even if it is delivered to you as object code. For example in your case, from https://os.mbed.com/platforms/ST-Nucleo-WB55RG/ follow the link https://github.com/ARMmbed/mbed-os/blob/master/targets/TARGET_STM/README.md